I don't know about articles, but the absolute best place for protection diodes is right across the power pickups of individual locomotives. Obviously, it's also the place that's the biggest PITA to install them! 
In general ESD protection should be close to the circuit you're trying to protect. If your TVS is at the transformer, when the train is 30 feet away and any circuit is interrupted that has inductance, the spike has to travel all the way to the TVS in order to be suppressed. The inductance of the entire path between the TVS and locomotive affects the ability of the TVS to effectively suppress the spike. OTOH, if the TVS is in the locomotive, any incoming transient is first met with the TVS protection diode.
