Streets of Moscow - Embarked bridging units

[PanzerTum]

Finally obtained a DV in Moscow at turn 23.
In my first effort I cleared the city west to east but that meant a long frontline which was split in two by the eastern counterattack.

My second try was from south to north, shorter and sturdier frontline which didn't go very fast but was more reliable.

I was wondering why the Soviet bridging engineers were all embarked on trains at the rail depots. It doesn't make sense for them to stay on the train when the enemy nears, so I was wondering what the thought behind it is.