They used to carry the Beretta 96 DAO in .40 S&W. Not sure if any still do.
Now they have switched to the Sig P227 in .45ACP. Not sure if that has changed.
Sorry, I don't know anything about backup guns and their policy. I'm sure it's not a big deal if a trooper wants to carry one.
Side note, one of my baseball coaches in Pony league was a state trooper and knew I liked firearms, so when he was issued his new Beretta, he made sure to show it to me. As a 14 year old, that stainless Beretta was a work of art. I still have an affinity for them... but I now know better than to buy a .40 (just kidding, just kidding... but seriously).