zee1ee

Prior to the current events, he pretty much accepted his fate. Remember: the blot is a pretty fatal illness with no cure and with the machine being well- a myth, he thought there was no hope and he was gonna die and he doesn't want to waste whatever is left of his life chasing after something that may just be a myth after all. But after getting a hold on the map, he found a will to live. To not die painfully and ending up like the others.

So when he disconnected the train car, it was a matter of inevitability. Dyle and Bendy may have gotten along with their love of trains- but that's just it. They don't know each other and Bendy doesn't seem like the type of guy to risk his life for people he just met, much less objects he doesn't care about (the cargo). If it was Boris, he would've tried harder, but Dyle is just a guy he just met. And before that- he told Dyle to leave the cargo behind!

The train car was already getting consumed by Mother, it was already a lost cause to begin with but Dyle still went with no regard for the rest of the train, the other crew, and even his own life! So when he got stuck, Bendy is introduced to two choices:

Try to save Dyle and risk dying himself when he's the only one who can see the map and is very crucial for this journey.

Live to see another day and go back with the others.

I'm sure it's clear that anyone in Bendy's position would have made the same decision. It's a fight or flight situation and he chose flight. And I'm sure this action will have consequences since Dyle is the only one that is keeping the train even functioning but I'm pretty sure he forgot about that with the adrenaline and it will come back later.