A simple physics editor would be amazing. So many variations between each unit in real life. It would be great to be able to edit various aspects of a loco / train to be a little more like real life. For example you may take a train out that has a bit of a dodgy engine, so is down on power compared to the norm, or one that has just been serviced so is a bit more powerful than the average. Something like a simple but global parameter editor such as 'Engine power' and then have a more/less in percentage. So we could say ' subtract 10% horsepower or the like. Worn brake pads? ( slide the gauge 10% to the left ) etc etc and save as a custom setup too What I don't want is something where you can go stupid like add 3000hp to an engine. That is NOT realistic and pretty boring. But being able to adjust acceleration and braking physics by 10-20% either way would be a fabulous addition.
This seems like a really good aspect for a simulator. On car simulations I've used you have the chassis and engine setup parameters to change for a particular circuit's character, for example.