The air in front of the portal would be pushing into the portal, and the only acceleration on it would come from the air behind it being pushed into the portal and any other air pressure differences between the two sides.
There’s no human pressure differences between the two sides of the portal, so there would no be any reason why you would suddenly be launched at 100 mph the way you would should the train have hit you. It would be like a tube or a hula hoop sliding over you at 100 mph.
The air in front of the portal would be pushing into the portal, and the only acceleration on it would come from the air behind it being pushed into the portal and any other air pressure differences between the two sides.
There’s no human pressure differences between the two sides of the portal, so there would no be any reason why you would suddenly be launched at 100 mph the way you would should the train have hit you. It would be like a tube or a hula hoop sliding over you at 100 mph.
As long as you don’t touch the tube you’re fine.