A train will not take power until every door proves shut, and proving shut is a switch in each door frame on a series circuit that the driver's panel reads as one light. At platform 2 the light goes out on door C3, on every unit, and the switch on C3 measures closed with a meter while the light is out.
I am a rolling stock technician on a fleet of suburban units, and door faults are most of my life.
How the interlock works.
Each door has a proving switch that closes when the door is shut and locked. All the switches on the train are in series, so the circuit is only made when every door is proved, and that circuit is what allows traction. The driver sees it as one light on the panel: interlock obtained, or not.
A door that fails to prove is a common fault and it is nearly always mechanical: a switch out of adjustment, a bit of grit, a door not quite home. You find it by walking the train and looking at the indicator on each door until you find the one that is not lit.
At one station, on platform 2, every unit loses interlock for a few seconds after the doors close, and the door that shows unproved is always C3, which is the third door on the third coach on the platform side.
It proves again on its own. The driver waits, the light comes back, the train goes.
Every unit. Thirty one different units on that route in the last year, all of them, at that platform and no other.
What has been done, and this fleet has a proper depot and a proper budget.
The switch on C3 on nine units adjusted, then replaced. The door mechanism on four units stripped and rebuilt. The wiring from C3 to the train line checked on two units end to end.
None of that could work, because it is not one unit. A fault that is in thirty one trains and one platform is in the platform, and there is nothing in a platform that talks to a door.
The platform. It has been surveyed for stray voltages, for electromagnetic interference from the signalling and the traction return, and for anything that could couple into a train's door circuit. Nothing above background.
The camber. A platform that is out of level can put a unit's body on a slight twist and a door frame with it, and a door frame on a twist can pull a switch off. It was measured. It is within the standard and the units either side of C3 sit in the same twist and prove fine.
What I did, which took a meter and a possession.
I sat in coach three of a unit at platform 2 with the door open on C3's switch and a meter across it, and a colleague in the cab watching the panel.
The doors closed. The switch went closed on my meter, which is what it should do.
The panel showed interlock lost, and the door indicator on C3 showed unproved, for about six seconds, and then proved.
The meter said closed for the whole six seconds.
So the switch was closed, the circuit through the switch was made, and the train line that the switch feeds was reading open, and the only thing between the switch and the train line is eleven metres of wire that I had checked that morning.
I have done that three times on three units and it does the same thing on all three.
What that means, as carefully as I can put it.
Something opens that door's proving circuit for six seconds, downstream of the switch, on a wire that tests good, on every unit, at one platform. And in thirty one units I have never once found where.
What is done about it.
Drivers know the platform and they wait. It costs six seconds per stop, which over a day on a busy suburban route is a few minutes, and it has been in the timetable's allowances for that station since 2020, which is when somebody in performance noticed the pattern and added dwell time without asking why.
The safety position is that a door which fails to prove and then proves is a system erring towards stopping the train, which is the direction it is designed to err in, and that is right.
What I do.
I do the door maintenance and I no longer replace switches on C3 for that fault, because I know they are not the fault, and I have written that in the fleet's known issues so the next person does not spend a fortnight on it.
The one change is that I do not ride in coach three through platform 2.
I used to, because it is where the fault is and you learn things by being where a fault is.
In October 2023 I was standing in coach three by door C3 with a unit at platform 2 and the doors had closed, and the six seconds started, and door C3's indicator went to unproved.
A door that is unproved is a door the train believes is open. The door was shut. I was looking at it.
What I felt for those six seconds was the draught you get standing next to an open door on a platform, cold, from the platform side, through a door I could see was closed, and on the seventh second it stopped and the light came back and we moved off.