New, faster and stronger locomotives were needed when SJ decided to raise the speed to 120km/h for express trains from Stockholm to Göteborg and Stockholm to Malmö. For such speeds the design with coupling rod between the wheels (which include D-loco) was inappropriate. The F-locomotive’s four engines running directly on the axles was the solution. Three experimental locos delivered in 1942 were followed by a series of additional locomotives 1945-49.

Information from “Svenska Ellok” and various other sources, translated by Google and a few friends!

After four years of electric traction on the Stockholm-Göteborg line, timings for the fast trains had been reduced by almost a third compared to the situation existing prior to electrification. The corresponding travel time intervals could of course be done on other routes, since electric power introduced, but the maximum speed on the new electrified lines was still 90 km/h.

During the late 1930s, it was decided that the lines between Stockholm and Göteborg, Malmö and Katrineholm, would be widened to double track, and at the same time, the maximum train speed increased to 120km/h. New locomotives were needed for this higher speed. Simultaneously the main line through Upper Norrland was electrified and in the early 1940s, it was anticipated that it would reach Boden. On the route Långsele-Boden there is the maximum gradient ratio 16‰, and some parts are rather curved. For the traffic on that route there was a requirement for new and more powerful locomotives.

For the new express train locomotives, the desired power would be for a train with a weight of 600 tonnes which could be driven with a maximum speed of 120km/h in the south and train with a weight of 480 tonnes with a maximum speed of 90km/h on the route Långsele-Boden. The locomotives’ axle load would not be allowed to exceed 17 tons. In the design and dimensioning of the locomotive, speed and power wise, it will take into account not only to the immediately preceding standing needs, but also to the requirements in the longer term.

When locomotives were specified for an upcoming major acquisition, there would be experimental versions which would be different both in terms of the mechanical structure of the electrical equipment and control system. In accordance with the loco committee’s proposals, SJ ordered in 1939, three express train locomotives with axle scheme 1’Do1′ of ASEA, ASJ Falun, Motala and Nohab. The new locomotives inherited the type description from the heavy express service trains steam locomotives previously used of the category F, which in 1937 were sold to Denmark. The locomotives were numbered 601-603. It was also decided that the three locomotives would be different for them thus to gain experience essential to differing technical solutions. Manufacture and assembly of the locomotives would be by the mechanical firms; 601 by ASEA & Nohab, 602 by ASEA & Motala Verkstad, 603 by ASEA & ASJ Falun.

All locomotives were formed on fully welded fixed frames as opposed to bogies. On all locomotives there were transmissions with hollow shafts (axles), which meant that each of them had a motor that drove a shaft, enveloped the drive shaft, and from which the torque transferred to the driven wheel through spokes from the axle ends. The locomotives were designed for 135km/h and the four engines could jointly develop a one-hour power of 2580kW (3500hp). In all locomotives the engines used by ASEA’s manufacturing were type designations KJ106 and KJ106A.

All locomotives were fitted with “Hildebrand-Knorr S” air brake systems. Brakes could be imposed on the three teams G, P and S (freight, passenger or express train). In the former mode there was an out-braking by 70 percent of adhesion weight; the S-mode, however had an out-braking a 175 per cent at speeds above 55km/h and 70 per cent at speeds below this limit. Each wheel was checked by double-sided twin blocks, that is, with four brake pads and brake cylinders. The brake was divided into two systems, one for the internal and one for the external drive axles, with separate control valves for each system.

For cooling of the engines there were four air handling units, each with two fans, driven by a motor. One fan was designed for the cooling of the rotor and the other on the stator. Moreover, there was a ventilation unit for transformer cooling with separate motor, and a circulation of transformer oil under its own power.
Some differences between the three locomotives are shown in the following summary:

Loco	601	602	603
Main Switch	AEG pneumatic breakers	ASEA contraction switches	ASEA contraction switches
Number of voltage steps	28 with ascending intervals	64 at constant intervals	40 at constant intervals
Number of driving modes	28	11	40
Operating power	DC	AC	AC

There were also mechanical differences, and not all wheel-bases were the same.

The three test engines were delivered 602 in April, 603 in October and 601 in November 1942. The first test period, when staff travelled, the locomotives were used mostly on the route Stockholm – Hallsberg quite successfully in test runs of various kinds. Later this was extended to the entire route Stockholm – Göteborg. Samples and measurements continued until the latter part of 1943. During the tests there was special attention on the selected accomplishments in terms of trailer bogie arrangement and suspension on their properties and their dependence. The Krauss-bogie (601 & 602) proved to bear no tangible advantages over tail-bogie and the different spring systems (three-point system) to be of little value in certain situations. This system had been applied to a large extent on German locomotives, but it required a careful adjustment of springs and spring balances.

In the case of the transmission, some problems were noted with Sécherons output elements of the locomotives 601 and 603, when the spring-driven ball rods were exposed to excessive wear due to the locomotives suspension movements. AEG-elements in the locomotive 602 was found to not wear so bad but on the other hand, there was considerable flange-wear of this locomotive because of AEG-large system inertia at the lateral movements.

The auto-transformers caused some problems, especially in locomotives 602 and 603. Voltage regulation system on the locomotive 602 also caused numerous problems in the form of short circuits and arcing in the fine-regulatory system, a couple of times resulting in head-breaker-explosions. Over the period when 602 was more or less shut down, the loco crew was not inclined to serve the locomotive.

Gradually, the three locomotives were rebuilt so that they largely came into line with the later mass-produced engines. Reconstruction of control system on the locomotive 602 was planned in 1948 but apparently it did not come into existence until a few years later. The engine went into the main workshop in Örebro from October 1953 to October 1955 for an initial conversion. Mechanical modifications, comprehensive framework and bodywork with this locomotive was extended and the fronts were changed in 1957. Locomotive 601 was taken in for renovation during the period February 1954 to July 1956 and locomotive 603 from August 1956 to May 1957. The rebuilding of the frontlines remained relatively straight.

In August-September 1943, contracts were signed between SJ and ASEA and the three mechanical locomotive-providers on the production of a series of 12 F-locomotives, numbered 621-632. Delivery took place from 1945 to 1946. The mass-produced locomotives were based largely on the locomotive 603, but differed on some points. Among other things, was the buffer beam so that it came to coincide with the rounded front body and the three front windows were replaced with two wider ones. The distance between the inner drive shafts were extended, while the distances between the tag axles and the outer drive shafts was reduced, resulting in an extension of the fixed wheelbase by 160mm. The length over buffers was increased by 30mm to 18230mm. Louvers on the sides were given a more balanced position. Spring Elements of AEG’s type was introduced and the locomotives were fitted with brake valves of the type that had been tested on locomotive 601.

Upside-regulation of the motor voltage in ASEA’s principle was retained, but were dropped from the auto-transformer and replaced with a two-winding transformer, whose secondary side of the parallel-connected motors were also connected. The control engine arranged for only one speed. The engines were of a refined type of designation KJ106B. The main Compressor switch of ASEA’s manufacturing and pantograph of a BBC type was used.

In the years 1948-49 there were delivered a second series of F-locomotive with the number 694-702. With these, the number of F-locos became 24. These locomotives differed in few respects from those previously supplied. A new type of air circuit breaker came into use. About the same time that the three test locomotives were modified to better conform to the production, there began a spring element made of rubber for use in power transmission between the hollow shafts and driving wheels. Engine Operation was also retrospective, so that motor contactor happened when the controller was taken to S-mode with the load tap in position 3. Thus was obtained a shift mode.

During the period 1964-68, there was a general upgrading of F-locomotives, much of which represented an improvement of working conditions in cabs. Window openings were modified and adapted for the rubber strip mounted window glass. Also the older relays were replaced with plug in relays. The number of sand boxes was reduced so that the sanding only came to be in front of the main driving wheels in each direction. Skid-brake was introduced and road brake valve was replaced with another type. For switching of the brake at about 55km/h there was introduced an electric actuator, controlled by a contact in the speedometer. The subsequent rebuilt locomotives became slightly easier than before.

F-locomotive service on the line Ånge-Boden was never on the scale which was assumed by the 1938 loco committee. For both express train and freight train service on this route there was obtained, upon recommendation of the loco committee, a locomotive with hollow axle and axle structure Co’Co ‘, which was type M (later Mg). The number of F-locomotives was thus greater than that which corresponds to the actual need.

The result was that they came to be used in trains, which could well have been the locomotive of another type. When F-locomotives in the late 1960s, were supplanted by the Rc-locomotives from express trains and express freight service on the triangle Stockholm-Gothenburg-Malmö-Stockholm, they were in fast and regional passenger trains on several other routes such as East Coast line, lines Stockholm-Oslo, Stockholm-Eskilstuna and Göteborg-Kalmar/Karlskrona.

The first F-engine to be abolished was no 699. The locomotive was damaged very badly in a collision near Kolbäck 1972. The remaining locos were delisted the years 1978-83.
In the table some measurements and weights of the test locomotives and the mass produced engines are presented.

Loco	601	602	603	621-702	All after conversion 1964-68
Total weight (t)	101.6	101.8	102.0	102.2	100.0
Adhesion weight (t)	69.2	69.2	68.8	69.2	68.0
Drive-axle load (t)	17.3	17.3	17.2	17.3	17.0
Trailing-axle load (t)	16.2	16.3	16.2	16.5	16.0
Fixed wheel-base (m)	6.7	6.7	6.7	6.86	6.86
Wheel-base 1-2, 3-4 (m)	2.45	2.45	2.45	2.37	2.37
Wheel-base 2-3 (m)	2.64	2.64	2.64	2.80	2.80