Question about MTU size and how packets are sent

Let's say I have 4 computers connected to the network, all sending and receiving sample amounts of data frequently, like in a game.

Questions: 1) If I were to adjust the MTUs of these 4 computers to something like 320 + overhead (yes I know that's stupid small), would the router send those 4 x ~360 within a single 1500 MTU frame out to the internet?

2) If so, does it have any benefits with something like congestion? I know what the disadvantages are already.

Reason: I figured out that a network issue is related to MTU size but then I had this thought.

TL;DR If I have this right, a single Ethernet frame has a MTU 1500. I imagine it as a train with each frame being a railcar. Can I put 4 different products in one railcar if they all fit?