The vanilla furnace minecart is replaced everywhere, including in worlds you already have, so existing carts pick up the new behaviour with no migration.
It no longer runs at half speed. It ignores slope slowdown entirely, so it climbs without bleeding momentum. It cannot be shoved by other minecarts, so a rear-ending cart cannot derail it. And instead of vanilla's decaying push vector it adds a constant 0.025 blocks per tick along its facing, for as long as it has fuel.
It burns anything. Any registry fuel works: coal for 1600 ticks, a blaze rod for 2400, a lava bucket for 20000 with the bucket handed back. The tank holds 32000 ticks and refuses an insert that would overflow rather than wasting it.
Powered rails are the on/off switch. Crossing a powered rail lights the cart, crossing an unpowered one puts it out. Fuel is preserved either way, so a station is just an unpowered rail.
Out of fuel it stops fast. An unlit locomotive multiplies its horizontal velocity by 0.75 per tick instead of vanilla's 0.98, so it comes to rest in a couple of blocks rather than coasting for a hundred.
A lit or moving furnace minecart automatically attaches any non-furnace minecart it overlaps, or that sits within one 1.5-block probe step behind the last car, up to eight carts total. Furnace minecarts cannot be trailers, so there is no double-heading.
Each trailer is dragged by running a ghost probe cart one spacing behind the car ahead and teleporting the trailer onto it every tick, matching position, pitch, yaw and velocity. The chain follows curves and slopes exactly rather than approximating them.
The cost of length is speed: the locomotive's maximum is multiplied by 5% per car, so a full eight-car train runs at 60%.
Below 100 ticks of fuel, the locomotive pulls one fuel item per tick from the first trailer, if that trailer is a chest or hopper minecart. Lava buckets are swapped for empty ones in place.
A locomotive with a coal-filled chest cart behind it runs until the coal runs out.
A trailer disconnects when it is removed, when it is grounded below 0.01 horizontal speed, or when its train tag expires 60 ticks after the locomotive stops refreshing it. Everything behind it drops off too.
Off the rails, any car that leaves the track and everything behind it switch to a yaw-aligned drag at the locomotive's speed, so a derailment stays in formation instead of exploding into eight directions.
Trains survive world reloads: trailer UUIDs and order are saved to NBT and the chain is rebuilt on the first tick after load. Going through a portal uncouples the train deliberately, force-loading chunks so the cars are not lost.
A rope is rendered between coupled carts, in the style of a leash. The locomotive broadcasts its ordered car list to nearby players every 20 ticks to keep that rendering correct.
A container cart with a nine-slot dispenser inventory that fires on powered activator rails, one dispense per trigger with an eight-tick cooldown. It uses the full vanilla dispenser behaviour registry, so projectiles, buckets, TNT, fireworks and armor equipping all work.
Sneak-right-click flips its firing direction, and coupling auto-corrects its aim so it keeps firing the same way relative to the train.
