Try the "DEL" key, if you haven't.
I tried a different locomotive, which appears to have a flashing beacon on the roof, but it does not have a way to manually turn it on.
Maybe some content does not have a function to turn it on manually. The G.E. unit wouldn't, but the E.M.D. did.
The key typically used for JR items or those using the tfx script is the number pad 1 or NUM 1, which is sometimes also the END key. This is the same as the pantograph key on an electric locomotive.
IRL many beacons like this were simply tied to the reverser lever, and when it was in fwd or reverse the beacon would be on so in Trainz they will be on when the locomotive gets moving (and for compatibility with AI drivers).
The key typically used for JR items or those using the tfx script is the number pad 1 or NUM 1, which is sometimes also the END key. This is the same as the pantograph key on an electric locomotive.
IRL many beacons like this were simply tied to the reverser lever, and when it was in fwd or reverse the beacon would be on so in Trainz they will be on when the locomotive gets moving (and for compatibility with AI drivers).