My thoughts: I got accustomed to the default key mapping* and I like it (was mostly because I figured if new stuff is going to be added to the simulator as in different engines / control types, then default would work better and going back would force me to retrain my brain if I went custom keys.
*I was contemplating to switch gearing and throttle so that key combo would follow their position and movement as the physical in-game movement. (Q+A throttle W+S gears) or even moving gears to number-pad -+ (left hand / right hand separation as in real life).
Only downsides that cross my mind of the current key assignment is that sometimes while outside of the cab (taking external screenshots) if I lift my finger off right click I could accidentally switch gears or throttle (hold right click to move enabled); and sometimes I bring up the occasional switch camera activation menu by accidentally right clicking.
There's also the SCROLL WHEEL and MIDDLE CLICK, which now are left unused. SCROLL WHEEL could be used for the ZOOM IN / ZOOM OUT function, opening up the LEFT CLICK to operate cab controls. Though the cab controls could also work as a key combination for extra safety like HOLD "keyboard key" + LEFT CLICK to eliminate accidental in cab operations.
But even if I had the option I would still use keyboard for train controls as reaction time is much faster than to operate in-game cab controls with mouse.
RIGHT CLICK to switch cameras is not that essential but I am to lazy to memorise the numbers for camera so I welcome its function somewhere. MIDDLE CLICK could replace RIGHT CLICK for camera menu pop-up.
Camera could also be moved to number-pad eliminating the RIGHT CLICK hold, but then some keyboards are ten-keyless, and distance from operational keys is kind of long.
"Option to invert vertical mouse axis" that's not for me as I am not a bat . :P
Hope it helps.