It seems like input handling has changed with 0.6 and does not use Windows key mapping any more, but scancodes directly, or it simply does not allow Caps as a modifier any more. I have mapped the Capslock key to serve as Ctrl key for ergonomic reasons (almost got RSI trying to play MMOs with the regular Ctrl key…), and I managed to make that work with the “old” Railroader version. Attempting to map Capslock+anything now simply yields a Capslock bind.
Yes, in Railroader 2025.1 the custom input binding system was rather limited – you had to use whatever number of modifiers the binding was initially set up with. Because of the way that system worked it was possible to get it to treat caps lock as a modifier (or really any key). The new system allows 0-2 modifiers per key and is much more flexible, but unfortunately this came with the cost of not being able to treat caps lock as a modifier.
You might look into a tool to remap Caps lock to Ctrl? From what I can tell there are some options for that out there.