Here are two tools for measuring distances on plans
or model railroads. The wooden one is useful
on actual roadbed. The pounce wheel is useful on plans.

I needed some custom curves on the O-Gauge portion of track. I used MTH RealTrax.
It comes in a variety of fixed-diameter curves. There were a few places where those stock curves
just didn't fit the needs of the plan. In some cases, I wanted transition curves to change
from tangent track without a lurch. So, I would cut two pieces of track to yield four partial
sections. Then glued an end from one to an end from the other to make a custom section that
was just right. It was also necessary to assure the electrical connections. Here is an example.
This technique allowed me to make shorter sections of curve or curve tracks with two, different
diameters.

From Daniel Lamarre on the FaceBook Group for Marx Trains - 2019-11-10
A tip for those who dismount 999 motors (and other motors).
To remove the assembly of the front truck, there are two
strong tabs you need to straighten. Instead of folding
again the tabs when remounting, I drill small holes in them.
After sliding onto them the front truck assembly, I only need
to insert a copper or steel wire in the holes. This makes a
second dismounting easier (and without risk of breaking the
tabs by fatigue). So whether you will have to dismount the
motor again or somebody else in 75 years, since Marx trains
are indestructible