| Thunder Mountain
| Basic information
| Map type
| File name:
| Map Info
| No. of Stages:
|| Daylight, raining
| Deep Water:
| Map Items
| Health Kits:
|| ×25 • ×24 • ×1
| Ammo Boxes:
|| ×7 • ×9 • ×21
Aw, now you don't look like a happy camper.
— The Engineer finding the Sniper's hidden camping spot
Thunder Mountain is a Valve-developed Payload map included with the Engineer Update, revealed on Day 4 thereof. It is the third official Payload map to be developed and released since the Heavy Update, and one of three Payload-based maps released during the Engineer Update, the others being Upward and Hightower (a Payload Race map).
Thunder Mountain is a multi-stage Payload map, like Gold Rush and Hoodoo. Unlike other Payload maps, Thunder Mountain features new gameplay mechanisms not used in earlier maps, including a bridge which must be raised in order for the cart to cross, and sloped tracks, marked with yellow and black caution stripes.
- First Courtyard: Just outside BLU spawn is a courtyard that resembles Gold Rush's Stage A, Checkpoint 1.
- Bridge: At the end of the First Courtyard is a small, tighter section of woodwork that leads to an unraised bridge. By pushing the cart onto the point the bridge raises slowly until it is high enough for the Payload to cross.
Details of stage 1.(Large file)
Details of stage 2.(Large file)
- Final Spiral: In Thunder Mountain's final stage is a large tower, which the BLU team slowly climbs over the course of the round until they push it into a hole in the center, detonating the cart.
Details of stage 3.(Large file)
- On Stage A, in the BLU spawn, the player may see some BLU characters in an area similar to the Soldier Training map, around a blackboard. They will occasionally taunt, albeit with no weapons, and will also occasionally do the Thriller taunt regardless of the time of the year. 
- On Stage B, in the RED spawn, one may see the RED team swimming in a pool.
- On Stage C, near the second checkpoint (third dot), off in the distance the Sniper can be seen near his camper van (the same one seen in Meet the Sniper), a tent, and a fire. He only appears after BLU team has pushed the Payload past the first checkpoint.
The BLU characters as seen on Thunder Mountain, Stage A, from BLU spawn.
The train as seen on Stage A of Thunder Mountain.
The swimming pool Easter egg as seen from inside the swimming pool as seen on Stage B.
Close-up on the Sniper Easter egg as seen on Thunder Mountain, Stage C. The camper van is not visible on this screenshot.
An aerial picture of Sniper's tent and camper van on Thunder Mountain, Stage C.
July 8, 2010 Patch
- Added Thunder Mountain to the game.
August 17, 2010 Patch
- Clipped off sticky outcrops for smoother movement.
- Clipped various exploit ledges.
- Increased environment ambient exterior light level.
- Stage 1
- Fixed hole in respawn room brushes in RED spawn.
- Shifted respawn times to favor BLU by two seconds.
- Stage 3
- Clipped rocks off by BLU's spawn exit that leads to building B. (allowed Snipers to snipe the entire area)
- Removed embedded displacement. (building B interior roof)
- Removed rogue clip brush from building A balcony.
February 14, 2011 Patch
- Updated the final stage to have the train begin receding after 25 seconds. (15 seconds on the final stretch)
- [Undocumented] Reduced the cart speed on the final two ramps in the final stage to 1/2 normal speed.
May 3, 2012 Patch
- Fixed a build-in-spawn exploit in the Blue spawn room for stage 2.
July 10, 2013 Patch
- Fixed gate visible through ceiling in stage 3
- Fixed clipping in stage 3 Blu spawn
- Prevented players from building in the cliffside kill zone in stage 1
- Thunder Mountain was originally teased on Day 1 of the Engineer Update, on the map of Badlands, an area that a number of game maps occupy.
- There is an actual mountain named Thunder Mountain in Sierra Nevada, California; however, Thunder in this case references the history of nuclear weapon development at Los Alamos National Laboratory (see also the mushroom cloud reference on the Engineer Update page).
- Occasionally, a blue logging train will pass by on the train tracks at stage A.