In many third rail powered trains, the third rail is usually located outside of the two running rails, so you can avoid touching it as long as you walk between the two running rails. With the one exception of a train being on that track, it is generally the safest place to be.

Also, a lot of places usually put a non-conductive cover over it to protect it. I’ve seen flagmen sit on it over deenergized third rails, but I wouldn’t want to risk it.