After being asked many times, at railway exhibitions, how the automatic tram stops on our model tramways work, I would like to outline the basics behind a very simple automatic control system which, in it's simplest form for one tram stop, only involves four cuts (electrical breaks) in the running rails.
With two-rail wiring each rail has opposite polarity, i.e. one is positive and the other is negative, but with live overhead both rails are negative and the overhead wire is positive. Figure 1 shows the simple wiring circuit from speed controller to tram.
When powered from the overhead wire, or two-rail, this system enables a tram to stop for a short time and then move off as the next tram approaches without the operator having any involvement.
When running trams from the live overhead, all wheels which have pickups are bonded (connected together) and both rails are also usually bonded, but automatic tram stops are the exception to this, see below for diagrams.
The motor in a tram powered from the overhead rotates the same way no matter which way the tram faces, so the tram has a front and if turned on the track will go the other way. This makes it possible to run two trams going in opposite directions from a single controller. Both Foxwood Park and Wearmouth run four trams, two running in each direction, from one controller.
In the following illustrations the overhead wire has been omitted, live rail and tram wheels are coloured red and dead / unpowered rail and tram wheels are coloured black and trams are represented by one axle only.
For emergency use I always add a switched feed from the controller to the short rail so I can override the automatic sections. This is achieved using a On-Off-On switch with the track connected to the centre contact, the controller to one end contact and the switch section to the other end contact.
Figure 2 shows the track wiring for an automatic stop / start operation where one tram moves up behind a stationary tram which then moves off while the following tram stops.
This arrangement requires four cuts to be made at A, B, C and D in the running rails and the long and short rails do not need to be electrically connected but they can be if this is desired.
This makes an unpowered section which can be whatever length is needed, usually AB is as long as possible and CD is slightly longer than one tram, to give it space to stop.
In Figure 2 as Tram 1 can run with only one rail live it continues past point A but stops when it gets to point C because both rails are now dead. When Tram 2 arrives at point A it acts as a switch and makes the long rail AB live so Tram 1 moves off on to normal live track. When Tram 2 arrives at point C the rails are again dead so it stops.
George Wilkinson used this method for the two automatic tram stops on his continuous Sealane O gauge tramway and there are two automatic tram stops used, one for each direction, on our Sunderland Corporation Tramway where a shuttle timer with diodes on the end sections completes the automatic running.
Figure 3 is a development of Figure 2 which allows the stationary tram to be started from a remote location and gives more flexibility of operation particularly where turnouts or crossovers are before the stop. The greater the distance between AB and CD the longer is the time between tram 1 departing and tram 2 arriving at the stop.
It requires one rail to be cut at A and B and both rails to be cut at C and D. The rail at AB and CD needs to be electrically connected but unpowered and can be placed wherever required. Again section AB is as long as possible and CD slightly longer than one tram, to give it space to stop.
In Figure 3 rails AB and CD are joined electrically so that Tram 1 stops at point C where both rails are dead, but when Tram 2 arrives on rail AB it again acts as a switch and makes rail CD live and Tram 1 moves off, as described above when Tram 2 arrives at point C it stops.
Peter Rose uses this method for the four remote stops on Wearmouth and there are two automatic tram stops used in the passing loop on our Sunderland District Tramway which also uses a shuttle timer with diodes used at the ends. An additional diode is used in the wire linking sections AB and CD so that the tram in the loop only leaves forwards.
Figure 4 is another use ot the remote automatic stop / start technique used above and shows the rail cuts and electrical link needed to achieve the automatic control when Tram 2 has priority.
When Tram 1 gets to point A it stops (because section AB is unpowered) until Tram 2 arrives at C where it acts as a switch and makes section AB live.
When Tram 2 passes D section AB becomes dead again ready to stop the next tram. The positions of AB and CD will need to be far enough away from the crossing to give adequate clearance for the trams being used.
This application was added as a result of an enquiry from the Contact page asking if the technique could be used to prevent collisions at a single track 90° (or diamond) crossing,
Figure 5 is a development of the remote automatic stop / start shown in Figure 3 but the two parts of the cut rail are located where the trams are going in opposite directions. it shows the 6 rail cuts and electrical link needed.
This arrangement is used to make Tram 1, which has stopped on a double track section at A, move only after Tram 2 has arrived from the single track at B. To avoid collisions cut B needs to be at least one tram length away from the turnout frog
Both trams are still controlled by the same hand held feedback unit. This method is used at the ends of the town section on our freelance 00 tramway Foxwood Park.