Heya,
I've just checked this out:
You can place exoskeletons etc. on the tank without any improvement. This is just how it is with the game and is just likely a Wube decision.
As with moving the locomotive equipment, you can only add equipment when in the fuel tab and you cannot take equipment out. Instead you have to pilot the train, and then open the grid via the button at the bottom left under the train icon (beside the player equipped weapons etc.)
I have reported the second issue to Wube as this may have been an oversight as they do not have a locomotive with an equipment grid in the base games so it's probably not even on their radar.
Cheers!