This larger stay alive unit is ideal for rolling stock power reserve although will need a bridge Rectifier from the track power so the polarity is correct.
The circuit has large solder tabs which makes it easy to solder just two wires onto it.
can also be used for DCC Locomotives to give a Large power reserve. and no extra components are needed, just a connection to Function + and the GND pin.
* please check this can fit in the space you have in mind