It will be number XXVII...and will contain three Republic titles:

THE GREAT TRAIN ROBBERY (1941)
UNMASKED (1950)
THE MAN IS ARMED (1956)

The last two are familiar to many of us, but the first is an intriguing rarity that stretches the concept of noir (but what else is new...) in the capable hands of Republic stalwart Joe Kane and has Bob Steele as a hero a few years before he played Lash Canino in THE BIG SLEEP.

It also has a borrowed plot from the man who brought you Sherlock Holmes, as noted in the IMDB review below...

Arthur Conan Doyle wrote an "impossible crime" story about a train that vanishes into thin air. A famous retired detective, who is unnamed, lends his thoughts to solving the case.
Two Republic westerns, The Great Train Robbery and William Elliott's THE LAST BANDIT, seem to borrow the basic mystery and its solution. If I'm not mistaken, a 1930's serial THE LOST SPECIAL also involves a missing train. I also remember seeing a very clever animated film in which Holmes solves a similar problem with pure logic at the American Film Institute when they did a series of Sherlock Holmes movies.
If you want to stretch a point, Banacek once explained how a flat car can vanish from the middle of a moving train.
But as far as I know, nobody credits Conan Doyle with the original story...