This Checkpoint is located a bit northeast of your safehouse. The main threat of this checkpoint is the swarmer Horde patrolling close to the MMU during at night. Make sure to at least tag a few of them with your binocular so you can track their movements.
The generator is located inside the fence beside the MMU. First, find the pickup truck parked uphill near the train tracks beside the MMU. The gas can is right beside it, on top of a some crates as shown below. Push the truck downhill so you can use it to climb up the MMU’s roof.
There are two speakers on the roof of the MMU while the last speaker is located in the guard booth along the road under the train bridge. Depending on where the Horde is located, you can disable this or leave it as is if you want.
Finally, get the gas can and throw it over the fence. Climb up the roof again then drop inside the fence to power up the generator to get inside the MMU.