Suggestion: Provide a new item. Do not replace the default loco item for those of us who may want to continue using solid fuels along side this mod. Thanks very much!

One of the point of this mod is to make the game harder though.

I see the point, though. I saw this mod and would like to add it in my existing game (which uses vanilla locos with solid/rocket fuel as well as some Electric Train locos for newer stuff), but if installing this will change all of my existing vanilla locomotives to use oil I'd have to spend a chunk of time immediately redoing my fuel distribution setup, rather than gradually transitioning as I add/replace trains.

Well, maybe I can put an option in config.lua whether to replace or not. Or, I could make a separate mod for non replace one.

+1 for this option

I also need this option

Check the description, I add new mod to support your idea, I don't want it to be an option in this mod since it will make unintended behaviour.