Valve releases a prototype image of the cart to be used in the forthcoming Goldrush gameplay format, along with a few details:

TF2 update: you may’ve heard that new achievements, maps, and unlockable items are on the way. The first map, Goldrush, introduces a new game mode. The BLU team must escort a high-tech mining cart through a series of checkpoints to the RED base within a time limit. The more BLU players near the cart, the faster the cart moves. The cart stops if any RED team member is in the vicinity. We’re balancing the map now; look for it within the month.

Source: Steampowered.com
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