I found this trying to make a mine cart launch up to a higher rail. Ididn't get very far because of this issue. It doesn't seem like it should be intended.

Build the setup in the photo, flick the lever, witness the slime block not come down.

Occurs with all types of rails. Also occurs when there is a full block there (which is intended) and I realized the rule is that if whatever is in place of the rail, if it can be moved by slime block, it will cause the block to stick, if not, then not obviously. While rails are allowed by the same rule as solid blocks, it doesn't seem like a slime block should stick to the edge of a rail.

Also, if the slime block is moving vertically with a rail on top, the rail will be destroyed and dropped as an item. This is where it doesn't fit the solid block rule.