Almost everything in railroad goes by at least two names, so the Knuckle Desk had lots of options for its title by virtue of the fact that there were so many terms and nuances in and around the point where two rails come together.
The whole area of the meeting rails is referred to as a joint, or more colorfully, the knuckle which we used for the name.
The devices used at the point where two rails are joined together are called different things - just to keep outsiders guessing, we suppose. Splices is a pretty popular term, and it's just a hair shorter than the two-word joint bars that's also fairly common.
Now, not to confuse things too much, but if you change the size of the rail - say we go from 100# rail to 115# rail then you'll need a special type of joiner. We often hear one of these adapters called a step-down (never, ever a step-up) or the ever-popular offset, or even comp bars for those with more of an engineering mindset.
Heck, even our Superintendent on the rails has two names. We call him Buddy most of the time, but he sometimes goes by his given name of Charles.
But you can just call this one the Knuckle Desk.