Not sure what you're asking ...

to go in a brake van

hi just want it as a stand alone circuit with a battery to work a light for either interior or for brake light led

but it comes on when van

but it comes on when van moves and goes off a couple of minutes after sotopping

Trains don't have "brake lights"

 You do realize that trains don't have "brake lights' that come on when the train stops?

a red tail light on the back

a red tail light on the back of a brake van That is red. I want it to be lit whilst train is moving and only turn off after a few minutes was stopped to save on batteries'

or another use for a tail

or another use for a tail light on a coach just dont want to add miniature switches to be on rolling stock would rather they have a motion sensor or vibration sensor to do that for you without being seen.

Capacitor?

Sounds like an EOTD or a FRED on the last car of the train.  If I'm understanding you want it to stay on as long as the train is moving and then go out  a little bit after power is removed from the train.  Shouldn't it be flashing though? At any rate a capacitor might keep it on for a bit  but not a real long period of time.

EDIT: I think part of the confusion might be the word "van". That's what we on this side of the pond would call  a boxcar, right?

Michael

..

A brake van is a caboose.

Andrew

OK....well wouldn't...

some type of capacitance circuit inside the caboose do the trick?

Michael

Would capacitance do the trick?

Sure. Just your usual voltage regulated passenger car lighting circuit, with a bigger than normal capacitor. You add a resistor in series with the cap to control how long it takes to turn off. Then you put a diode around the resistor so it turns on quickly.

would this turn it on once it

would this turn it on once it moves

I believe train tech do them

I believe train tech do them but wanted to have a go at doing them myself. As they are around £25 each. They can be connected to strip less for coaches too.

KISS

Maybe a battery with a reed switch would be simplest. Either make a magnet wand or drop the van over a Kadee magnet between the rails and shut off the marker at the same time?

Neil Erickson 

Still think capacitance is the way to go....

Once the track power hits it, it's going to come on and stay on and then when the power is interrupted a capacitance circuit as described by Pelsea will allow it to stay on until it bleeds out whatever charge is there.

To me at least, that is simple. Batteries, latching reed switches and magnet wands are something I would consider for a whole string of passenger equipment (I have) but not for an application like this.

Michael

changing whhels to contacts

then I would need to change any wheels that are plastic for metal insulated ones. I would prefer to add my idea that way I can add it to any coach brake can or caboose or even the engine. Then once it stops it can Sta on for a good 4-5 minutes