Programming a Pristine World
Rail, 24 bytes

$'main'    #
 -[world]o/


I think this works. And it's both short and readable (as far as Rail goes).


If the removed substring includes any part of $'main' we get Internal Error: Crash: No 'main' function found.
If the removed substring includes the #, there is no way for the program to exit cleanly, so it will always terminate with Crash: No valid move. I believe that it's not possible to delete a substring such that the track forms a valid (infinite) loop.
If the removed substring is in front of the #, it will be disconnected from the end of the track, so the train will crash (with the same error as above).
If the removed substring is after the #, it will also disconnect the # from the end of the track (and potentially even the beginning of the track from the entry point $). So again the same error.


As for the actual program: every Rail program needs to have a $'main' (or a longer variant, but we're golfing here) as an entry point into the function and the train starts on the $ going South-East. Everything on that first line can be part of the track though, so removing the 'main' the track is:

$          #
 -[world]o/


The - and / are simply pieces of rail which we need to let the train take those 45° turns.

[world] pushes the string world and o prints it. # marks the end of the track - the only way to safely terminate a Rail program.

Interestingly, this solution is only possible because Rail allows the tracks to go through the main line - if that wasn't possible # would be after the first newline and the code could always be shortened to

$'main'
 #


which is a valid program that doesn't do anything. (Non-space characters between ' and # wouldn't affect that.)

Also quite interesting: if I had just golfed the task printing world it wouldn't have been much shorter or simpler:

$'main'
 -[world]o#