Red lights at older steam engines - how to model?

[modelsof1900]

I'm reworking an older steam engine and there is a question about use of red back lights.
The tender has one great white lamp only for reverse running - electric lights, not old petroleum lamps.
I would like to see a red light at tender if engine is running in forward direction without a train - thus the engine only. Was there used a simple red glas blend that was set (plunged) ahead the lamp front at the tender for such a case? Or how was marked the train end at an engine?
I would like to insert a white and additional a red led into this lamp and all could be solved? However would be this correct? Unfortunately I have not a second lamp or specific back lamps to realize a red light at this loco.
What can I do in order to get a correct realization? The engine is a really expansive and factory painted model so that I do not like to make big changes.
If the new electrical pickups at tender are ready I will show a few first pictures here. Thanks for your help!