We're cutting a two dome tank car for another crazy project.
There was no way to mark the car with the walkways etc. that were cast on the car.
I tied a zip tie around the car where I wanted the cut.
Placed the car on the table and made the tie vertical compared to a square on the table.
Pulled the tie REAL tight and was able to use it as a cutting guide.
Make sure you get the fat end under the car or you can't cut close to it.