>>527323893
>Why can't I put them on one side sometimes though?
>>527331763 is correct. Placing a rail signal on one side of a rail makes that rail 1-way. From there, you can only add signals on the same side as the existing signal, unless the signal on the other side is directly adjacent, as indicated in white. Once that opposing signal is in place, the rail is 2-way, and you can place signals freely on either side.