The phrase "bottom rail" typically refers to a person or group who is at a disadvantage or in a subordinate position. It suggests being in a lower or less favorable position compared to others. Full definition
Top and bottom rails ensure a smooth and easy hang. (pbteen.com)
On the other hand, that could be enough space for your little one to get foothold on the horizontal bottom rail of the gate when trying to climb over the gate. (thankyourbaby.com)