That's a valid concern, but I don't think so. Some things to consider: 

• Resources are already stretched pretty thin with the actual war going on. There's going to be a moment when there's no money for repression machine. That wasn't realistic before, but now, with various sanctions, it is.
• From the "After Putin: Lessons from Autocratic Leadership Transitions" paper: "Though authoritarianism typically continues when authoritarian leaders exit, one positive note is that repressive conditions tend to ease in the five-year period afterwards".
• There's a difference between sabotage and terrorist tactics. The latter do not really work except in a "we will force you to spend a lot of money on security" sense. The former not just "sends a message" -- more importantly, it directly denies resources and makes governing harder.

Additional viewpoint that might be relevant:  if you believe the duration of the war to be mainly time-dependent (e.g. "everything will end the day Putin dies or the day Ukraine gets a shipment of modern weapons") instead of resource-dependent (e.g. "everything will end the day there are no Russian volunteer soldiers anymore") then each railway delay means some amount of civilian lives saved.