The ‘slip coach’ is an idea taken from the days when railways were being constructed. Before any train ran. The wagons carrying spoil to the head of an embankment, to drop the load over the edge, had a ‘slipping lever’ to uncouple the horse from the wagon. The horse stepped sideways and the wagon could be pushed the last few feet and tipped. When trains began running, those being ‘gung-ho’ days when risk assessments were not much thought about, the fireman – on some railways – would climb over the coals and when the driver eased off speed, the coupling went slack and the fireman could then lift the coupling off the hook. The engine then accelerated and the pointsman switched the track into the depot and turned the points back again to send the train into the terminus station. This practise caused a fatal crash at Hull on about 1840 but the slip coach idea was continued. The GWR is most famous for using them but most major companies had slip coaches, some more than others. It was a very dangerous practise when the only brake was the guard’s handbrake and safety depended on the main train not slowing down after the slip had been made. With the introduction of a powerful vacuum or air brake it was less dangerous but still dodgy – in my opinion. The GWR had a printed instruction to level crossing keepers in its ‘General Appendix to the Rule Book’ ordering them to remember which train had a slip coach and not to open the level crossing gate after such train had passed until the slip coach had also passed. And thereby hangs a tale, I think. In spite of the obvious danger of having a loose coach whizzing along at a respectful distance behind the main train have only heard of one slip coach collision. That was at WOodford Halse in 1935. The coach was slipped on a foggy day – and maybe the guard ought not to have slipped – but he did – and then the main train slowed down, he caught up with it and, because of the reduced visibility by the time he saw it he could not stop in time.
Before 1914 the GWR had 72 coaches slipped every week day. During the war slip coaches were discontinued and after the war nothing like so many were put on. The classic slip coach train was the GWR’s ‘Cornish Riviera Limited’ which left Paddington at 10.30 a.m daily with 3 slip coaches – one for Weymouth, slipped near Westbury, one for Minehead, slipped at Taunton and one for all stations Exeter to Newton Abbot, slipped at Exeter. As the spring came and Easter, as the holiday season got going, the slip coach would have an ordinary coach coupled behind it and maybe the slipped part of the train was 6 coaches long.
Each slip coach did not have more than one coach behind it because of the technicalities of the vacuum brake system. In the summer it was often the case that there were more passengers than could be accomodated in two carriages to the noble GWR put on an extra train – usually 6 coaches (try doing that on our ‘world-class’ railway) in place of the Weymouth slip. That would then run as a ‘part’ of the main train – i.e ‘10.30 Paddingon, First Part will depart Paddington 10.27 a.m’ or ‘at 10.33 a.m’. It was normal at peak times of summer travel to have the main train and three or more ‘parts’ – one for Weymouth, one for Minehead, one for Ilfracombe. These ‘parts’ had their timings in the Working time table ‘Suspended’ and then when the train was to run it was shewn on the working notice for that week ‘10.25 a.m Paddington WILL RUN’ written like that. It was brilliant railway. In spite of all the so-called ‘inefficiencies’ of the equipment they really tried for the passengers.
I rode up from Plymouth several times a year for 2 1/2 years in the slip coach of the 8.30 Plymouth, slipped at Reading about 1 p.m.
In all cases where there was a slip coach shackle was placed over the ordinary draw hook of the last coach of the main train and the vacuum pipes were connected without the self-sealing adaptors. The coupling altered at the last stopping place. The 8.30 Plymouth ‘adusted the sip couplings’ at Westbury. This meant putting the shackle of the last coach of the main train over the draw hook of the sli coach and self-sealing ends fitted to both vacuum pipes – on the slip and on the main train. The guard of the main train walked through the main train nearing Westbury and inquired over every passenger if they were travelling to Reading and if so to get out at Westbury and walk back to the slip coach. Then he came to the slip coach and asked if there was anyone on board for Paddington – in which case to get out and go forward into the main train.
Up until 1953 I regulalry saw the 8.30 Plymouth and the 8.20 Weston-s-Mare slipping at Reading at 20 mph. The whole train would be sent through the Up Main Platform (No.5) Loop and at that speed the guard did not pull his lever until he was just on the platform end. We boys used to crowd to that end to catch a glimpse of him doing this. Moving out to between Didcot and Swindon I did on one or two occasions stand on Foxhall bridge and see the 8.30 Weston go through at 70 and then the slip coach appear after a minute or so all by itself, rolling along! An amazing sight.
The slip coach coupling had a draw hook that was hinged at the base and when the pointed tip of the hook when lifted up into the normal position it had an iron casting an inch above it. Into that space fitted a wedge, filling the gap and holding the hook in place. In the slip guard’s compartment there was a lever which stood upright between two guide bars. When pulled backwards it pulled the wedge back allowing the hinged hook to drop. Pulling the lever back also turned a rotary valve which then admitted air into the vacuum brake cylinders of the coach and the coach behind if there was one. So the slip coach was braking and dropped back from the main train. The guard then pushed the lever fully forwards. This rotated the said valve and allowed the air pressure which was then in the brake cylinder to depart into a large vacuum chamber under the carriage. The brakes were no released and the coach free-wheeled happily along. All that was then required was for the guard to pull the lever back and apply the brakes as he coasted into the station. Therefore the important thing for the slip guard to do was to slip his coach at the right distance from where he had to stop.
slip brake valve
Diagrams from GWR General APpendix to the Rule Book. 1936

Diagrams from GWR General Appendix to the Rule Book. 1936