Setting up and "dropping" the weld takes time. The track must be cut, cleaned, and allinged before the thermite can be sparked off and drops to fill the void. This takes time, normally about 2 hrs.+ per weld in the field.

After the weld has beed dropped and coold a little bit, the rail must ground back to smooth.

If you google it you can see people setting up the weld. There are molds that go around the prepared area to contain the thermite. The Powerdered thermite is then set in the cruicible and sparked of turning into a liquid. Once molten it drops down into mold and fills the gap.

In CWR often a rail streatcher or heater must be used to expand the track and maintain the Designated Neutral Rail Temp.