To make these trains, you would take 2 or more Minecarts and link them up using a lead. You can link up to 10 Minecarts at a time. The first Minecart leads, and all other Minecarts would be pulled behind it.
However, the trains would require more speed than a single Minecart to go up ramps.
To disconnect the carts, break one cart. Any connecting leads will break and drop as items
I originally though that this was going to be some crappy suggestion that is completely overpowered, but this suggestion is actually completely balanced and simple. Not too complicated, and not too OP.