The would be moving at 10 meters per second in regards to the train that didn't touch them, the same as the were before the train got close enough to touch them.

Think of it the other way. If the went into the stable portal and came out I'm from of a moving portal, what would happen?

The portal would move forward and swallow them up and spit them back out the way they came in.

They would not have accelerated in the process. They wouldn't fly out the portal they just walked in at the speed of the train. The train didn't touch them so it can't transmit any of its momentum to them.