Step 1: Draw a combat map of 2-square-wide mining cart tracks, seperated by 1 square in between.
Step 2: Hand your PCs a couple of mining carts.
Step 3: Profit.
When running encounters with these vehicles, it's critical to keep things in relative motion and distance from each other. The base assumption is that all of the carts and their occupants are flying down the tracks at high speed. The important part, and the only part you need to map out, is where they are relative to each other. In this sense, upcoming hazards at the tracks are more coming at the carts, rather than the carts speeding towards the hazards. This isn't any new advice here or anything, the chase scene idea done this way is pretty old, but I thought I would be remiss if I didn't mention it.
Mining Cart Large Vehicle
AC 3; Fortitude 10, Reflex 3
Speed Up/Slow Down + At-Will (1/rd.)
Effect: Move the mining cart 3 squares in either direction on the track.
Lean Over + At-Will
Effect: You lean the cart over to get a little closer to the enemy. The reach of your melee attacks increase by +1 until the start of your next turn. At the end of your turn, each occupant of your cart must make a DC 10 Acrobatics check or fall off the cart.
Switch the Tracks + At-Will
Attack: Make a ranged basic attack against AC 17, Reflex 15 or Fortitude 15.
Hit: One mining cart of your choice moves laterally to a track adjacent to it.
Ram + At-will
Trigger: The cart ends its turn adjacent to another cart.
Effect (Free Action): Both carts takes 4d10 damage and all occupants are subjected to the following:
Attack: +9 vs. Fortitude
Hit: 1d8+5 damage, and the target is knocked prone.
Fall Off + At-will
Trigger: You would be pushed, pulled or slid out of the cart.
Effect (No Action): You stay in the cart, and hang on to the edge for dear life. Your feet scrap against the fast-moving tracks as you struggle to get up. You take 2d10 damage and are knocked prone in the cart.
Trigger: The cart drops to 0 Hit Points or less.
Effect (No Action): All creatures in the cart are thrown out, take 5d10 damage and are knocked prone. The surviving occupants of the cart then move onto the next combat encounter.