Notch is adding mine carts to the game, but I have a few suggestions;
There has to be a vehicle powering it, so steam engines should be added as well
You could put together on by a little fold-out menu with different parts like chassis, boilers, etc.
these would have different stats attached, like speed and power.
You can drive the train if you want, if not they could be automated.
Train tracks, X and Z size is doubled while Y is identical. Crafting recipe:
Replace wool with iron blocks. 32 crafted at a time.
Wheel, a crafting ingredient.
Sixteen crafted at a time.
Train wheels, the base of any train. Size is 2x1x2.
Replace slabs with wheels. Build on these to make your train.
Battery, used to power electric trains.
Replace red wool with redstone dust. Batteries are one of the two ways to power trains, the other being a furnace (meaning you cannot smelt anything if the furnace is in the same cart as train controller). No need to refill, but more expensive for crafting.
Train controller, which is what makes a train a train.
Replace red wool with redstone dust, replace diamond with battery. Activated in similar way to controllers from the zeppelin mod, except a lever on the top of the block activates it. Also controlled using numpad (8 to go forward, 5 to brake and 2 to go backward).