You can add this tag in your nightmode settings if your locomotive has any such setting in its configuration and that is what these do below.
From the DevWiki, which is a pain in the proverbial to find what one needs:
nightmode
Only add this tag if you reference a default-night mesh in the mesh-table. It is mandatory if you want a night mesh to show. Values: home, lamp or constant.
Home - switches on night effects at dusk and off sometime during the night.
Lamp - switches the night effects on from dusk to dawn.
Constant - lights are on day and night.
So I guess, if you already have "Lamp" in your locomotive's configuration, change this to "Constant" and it will do just that. IIRC in TRS04 one could even set the time of day/night this was activated but as said, I could not find this in the DevWiki.
Cheers
VinnyBarb