All reference frames are valid - you could say the Earth and the people are moving and the train is stationary, you could say the train is moving and the earth and people are stationary, or you could say they each have a vector moving around the sun or anything else

But when you travel through a portal, the only valid reference frames are you and the entry portal. Your momentum relative to the Earth doesn't matter - why would it? You can open a portal to the moon and jump through, and we see momentum is preserved. The Earth isn't a special reference frame, it's just the most noticeable one.

So let's pick the reference frame of someone on the track. Let's look through the portal and say there's a sign on the other side - as it approaches, you'd see a sign approaching you through the portal. Relative to you, through the portal the sign is moving at 30mph. The portal passes over you - you haven't moved, but you enter a new reference frame, a frame in which the Earth and everything on it is moving at 30mph