CANAN & WMSPT. is Canandaigua (NY) & Williamsport (PA). That's the RPO route and not the name of a railroad, if anyone was wondering. Mansfield happens to be fairly close to Williamsport.
Here's an earlier version of the cancel on the back of this cover:https://www.stampcommunity.org/topi...PIC_ID=28101
interesting reading there