If you remove train destination it stops momentally.
If it has wagon and you remove locomotive, wagon starts momentally with high speed.
Video: https://screencast-o-matic.com/watch/cq1VXVT09k
The problem was that the train still had its speed, it just didn't move (the code to move wasn't called) in the state NO_SCHEDULE, so when the locomotive was mined, it just got to a different state (MANUAL_CONTROL) and started moving again.