Underground Unit Biome
Underground Unit is a secret biome that features a lot of unique puzzle elements.
Important! Unlike other biomes, leaving Underground Unit will not level up your engine. I.e. the max number of wagons does not increase, and any engine specific power does not level up. For this reason, and that the Underground Unit biome is the hardest biome, Underground Unit should be your last biome to visit before ending the game.
Connections
Entering
Underground Unit can be entered from any other biome by going through a tunnel under mountains. At most one tunnel can spawn per level, and the spawn rate is pretty low on your first play through. However, a tunnel will spawn on every level when you visit a biome for the second time.
Important! For reasons stated at the start, you should carefully avoid the tunnels on your first play through until you have a strong train.
Leaving Via Boss
Unlike other biomes, unlocking the boss level requires having 5 
Key extension. This is yet another reason why you should avoid Underground Unit early on.
Beating the boss brings you to the Secret Lab Facility Station instead of the usual Biome Junction Station. This station offers the 
Brake Cartridge, but the only path you can choose is 
UnRailD HQ (Another reason why you should avoid Underground Unit early on).
Tip: you can recycle some special wagons that you don't need at a 
Blacksmith Stationi, and then collect multiple keys on a wagon stored in the storage.
Tip 2: Wagons printed by 
Wagon Printer has one additional 
extension slots, and thus can hold one more key.
Leaving Via Portal
If the player enters the biome without sufficient number of Key extension, the player would have to leave the biome via one of the portals.
Portals spawn on every level after the initial level. If you do not intent to leave the biome, be sure to check for portals when building tracks below a mountain.
Leaving the portal brings the player to a mini Biome Junction Station where the player can only choose the next biome following the biome they entered from.
Since you would have practically skipped the boss fight of the last biome, the game punishes you by not giving you any cartridges and no engine upgrades.
Environment
Other than having various puzzle elements, the environment is very similar to that of 
Monorail Meadows.
Bridges can be built using wood.
Unlike other biomes, the stations are initially locked behind a force shield.
To unlock the station, the player must place a puzzle box on top of a red button. Once complete, the box and the button, along with the shield around the station, would vanish at the same time.
There is only one real red button per level. If you see two, one of them is the hostile robot B.U.T.T.O.N..
Laser Turret
Underground Unit is full of laser turrets firing lasers reaching across the whole map. Each turret has 15 hp and can be taken down by axe or pickaxe.
Lasers fired by the turrets can block all movements, and therefore are the number one threat of this biome.
Although lasers cannot block track placement nor the movement of the train, the train itself does not block laser either.
Lasers are only blocked by mountains, trees, rocks, other laser turrets, and the puzzle box. Notice that out of all the items listed above, only the puzzle box is the one that can be moved by players.
It is entirely possible for players to soft lock themselves if they blindly clear all resources on the map (e.g. using dynamites) where all laser turrets on the map are blocked by the lasers of other laser turrets. It is therefore very important for players to plan out how to clear the resources. E.g. a tree that's blocking a laser should not be chopped.
NPCs
G.R.A.B.I.E.
Geo-Resource Acquisition Bot for Item Extraction. Rideable and milkable.
G.R.A.B.I.E. is a semi-hostile 
milkable robot with 20 hp.
Although it does not damage the players nor the train, it can steal your tools.
Players can dash into G.R.A.B.I.E. to force it to drop the tool.
In some situations, G.R.A.B.I.E. from the last level can take your tool outside of the screen, making that tool permanently unreachable.
Like players, G.R.A.B.I.E. cannot pass through laser.
When G.R.A.B.I.E. is holding the bucket near water, the bucket can be fill.
B.U.T.T.O.N.
Booby-trap Utility That Triggers On Nearness
G.R.A.B.I.E. is a hostile robot with 10 hp that looks nearly identical to a red button when it is stationary. However, if you zoom closely, you can see that it has 4 black legs.
When it is freed from surrounding blocks, it will chase the closest player and self detonate in 5s.
Tip: when mining towards a red button that you can't tell if it is real or not, mine the block at the corner first and try to hit the button at 45° with the tool. You can kill B.U.T.T.O.N. without it self exploding as long as it is not freed.
Boss Fight
Dijkstra
The Administrative Intelligence of the United Railway Division. Ensures that every crew reaches peak performance, by eliminating everything that's slowing them down.
Unlike the other bosses, this boss has two phases. In both phases, all the resources are pilled at the bottom with a line of water at the top.
The boss moves across the top with one eye firing a beam of red laser and occasionally firing tracking torpedos.
Unlike blue lasers, red lasers do not block player movement and they kill players instead. In addition, red lasers can be blocked by the train as well. This makes the train a good hiding spot for players near the train.
Bots can dash through the red lasers without dying when they are about to be hit. But players will always die when dashing towards the laser.
In phase one, the path is blocked by several blue lasers that can be deactivated by placing the puzzle box on top of red buttons.
In phase two, tools are respawned near the train and a new boss dialogue is played. Several rotating red laser turrets are placed along the middle of the road. They can be deactivated by placing the puzzle boxes on top of red buttons as well.
Although you can hit the red laser turrets using tools, they cannot be truly destroyed this way.
Since the turrets are constantly rotating, it is possible to not deactivate any turrets and just carefully dodge the lasers while build tracks straight to station.