This video was not created as a guide.
This is an alternative method for 250k zombie kills on the railroad tracks. My video (at 40:10) shows me using a Treadmaker, but you can probably use anything you prefer.
This is probably not more time efficient than the Slingrot method.
I prefer this because I do not have to worry about cars/obstacles in the road or people shooting at me or accidentally making a wrong turn and making a new checkpoint.
In Case 3 when you need to Meet with Dr Blackburne, (this might be able to be done sooner, but you will need to obtain the Treadmaker blueprint only by going through the story, in Case 5, if you want to build Treadmakers). You CAN just buy a Treadmaker from the shelter and have that on you when you create a checkpoint nearby the tracks, preferably at 100% vehicle health.

In this video, which I will recommend starting at 40:10 timestamp, simply shows me:
-spawning in,
-creating a Treadmaker from free vehicles on the map,
-driving to the railroad tracks
-clearing out the tunnel horde
-driving to the end (where my Treadmaker can no longer pass through)
-turning around and driving to the underpass before the yellow train engine-car, turning around again and repeating.
That is what the video at 40:10 shows. However, if you want only zombie kills and dont care about the PP, you can farm the "tunnel horde," turn around and drive through the underpass, and go back and farm the tunnel horde. This would be more efficient than driving all the way to the end and back.
-The underpass before the yellow train engine-car is about the furthest you need to travel in order for the tunnel horde to respawn. You do not need to drive up to the yellow train engine-car or pass it.
-If you are using the Treadmaker, approach the actual metal tracks at an angle so you dont get stuck temporarily.

That's pretty much it.
The video also explains that while you are doing this you can complete all or some of the 100 vehicle kills, and all or some of the 500 fresh zombies. However, I recommend doing 500 fresh zombies at the mall when there is a short cut-away scene of 5 fresh zombies in the parking garage, that way you can have keyboards and gloves for those challenges as well. But for 100 vehicles this would be a great time to do those, since on the way to the tracks there are about 4 vehicles, and on the tracks themselves there are at least 3, which do respawn on the same load.
- I HIGHLY recommend AGAINST trying to kill 2 birds with one stone and trying to get PP through 1000 or 2000 hit-streaks at the same time while using this method. There is another way to get easy PP where you can easily get 2000 hit-streak for the 10kpp which is more time efficient and easier AND faster to pull off while using the vaccuum powerup or ice powerup.

TRUST ME, for farming PP, it is easier AND FASTER, and you will save MORE TIME using exosuit vaccuum or ice powerups than trying to combine the zombie kills and hit-streak PP here, when using vehicles.
This video shows me trying to get zombie kills AND hit-streakPP. Don't do this.
JUST farm the tunnel Horde and reset it by driving to the overpass with the ATV and dirtbike before the train engine