train xl 80 unit fires up then shuts right off
I just yesterday dealt with one like that. It had electric ignition (glowing ignitor) and a single yelloow "flame sense" wire. This is only the second one of these I've found in a decade, but the problem and the solution is the same. IF your unit uses ion flame detection, the burner ground has become oxidized. It may look like metal against metal, but a fine coat of invisible oxidation had formed between the metal parts. It needs a good "ground reference" before it can detect the ions. 

To fix this, I removed the burner. While I had it out, I scraped the flame rod tip some but it wasn't really dirty. When I put it back in, I intentionally made one of the self-tapping sheet metal screws dig a new groove instead of putting it back in the first grooves. I also got it good and tight. 

Just as in the first case, the mere act of disturbing the mechanical assembly caused a better ground to be formed. It must be working, because I didn't have to get up last night and go fix it.