Gun Runner suggests that if you have an engine open for work it is a good time to install a TVS between hot and ground. [Hot is a wire to the center rail pickup, Common or ground to any suitable place on the chassis]. Failing that opportunity one can install TVS where railpower is connected at the track or at the transformer output. I long ago developed hand tremors and don't even consider opening an engine. I installed my TVS at various railpower distribution terminal strips where a jumper runs from the strip up to the soldering point on the rail flange above. A master railpower distribution strip (2nd photo) that sorts out wire runs just after exiting the transformers out to separate power districts is a good place for TVS. Also, as noted above, on satellite terminal strips which have jumpers leading up to rail soldering.
Below: Plate jumpered Satellite Terminal Strip equipped with TVS that provides protection for 5 potential railpower feeds. Green is Hot and code for the particular power district. White is Common. wires are from a 180 watt POHO transformer linked to a TPC [ track power controller].
Below: Master Distribution Strip routing railpower from transformers to districts (TVS not installed in this 2010 photo)