PC Scenario Can Be Created But Not Played Unless "off The Rails" Is Enabled

Hi, I took me quite some time to understand why I could create some scenarios that were not playable in the end. For example on "rapid transit" route (weird scenario idea but good for demonstation purpose) : If you put a BR101 to stop at Leipzig (tief) which is a local train sbahn station, it lets me pick Leipzig tief as stop, because the length of the train fits, but not the kind of train apparently. So I end up waiting in front of a red signal forever.
Wouldnt it be better to show a warning in the scenario designer when the scenario is just playable if "off the rails" is enabled?! With "off the rails" enabled the red sign finally switched to green.

What are the criteria that qualifiy for "off the rails" needs to be turned on?
I just came across that by incident, because on Main Spessart Bahn I wanted to create a Intercity service from Gemünden to Aschaffenburg.

I know those IC services dont run this route, but what if I want to create a re-routed service that needs to take this route, for entertainment purpose? I just checked if all of that route I want to take is electified, seem so. So what prevents me to create the IC service there?