In a bar, the owner selects certain brands of the various types of liquor which are either inexpensive for the owner, or which he or she believes will confer prestige on his or her bar. These brands are then placed in a bottle holder, a "speed rail" at the edge of the bar so that as the bartender faces the customer, he or she has the bottles immediately to hand. Because they are so conveniently placed, drinks can be mixed with them rapidly, hence "speed rail." Those are rail drinks.
But if a customer wants a brand other than the brand used in the rail, if it is available, it will be displayed behind the bartender, usually in an attractive manner, so that the customers may see those brands and select from them. Those are call drinks.
This is a speed rail. The bottles have pour spouts on top, and the bartender stands immediately behind this speed rail as he or she serves, and so can make a drink rapidly with these brands of liquor if the customer doesn't specify a different brand.
Here is a typical display of liquor behind the bartender in a position where the customer can easily see it. (Most displays contain far more brands than are shown here.) If you look at the top row, on the right, you can see that for the type of liquor known as Scotch whiskey, this bar offers Johnny Walker black label (considered a very high quality brand), Johnny Walker red label (considered a high quality brand), Cutty Sark (a good brand, preferred by many customoers) and Dewar's (a blended Scotch whiskey of the very highest quality). If a customer doesn't want the speed rail brand of Scotch whiskey, they can see what else if offered, and make a selection from them.