When you add stairs, stair railings are added automatically. You can modify stair railings at any time after their creation.
If you are working in a plan view, it may be helpful to use the Tab key to select the railing.
The railing line is selected, as follows.
You are in sketch mode, so you can modify the shape of the line to suit your design. The railing line can consist of connected straight and arced segments, but it cannot form a closed loop. You can resize it by dragging the blue controls. See Controls and Shape Handles and Splitting Elements. You can move the railing line to a new location, such as the center of the staircase. You cannot draw multiple railings in the same sketch session. For each railing you sketch, you must first finish the sketch, before you can draw another railing.
Modified stairs with center railing
If you extend stair railings (for example, to extend onto a floor), you need to split the railing line so that the railing changes its slope and meets the floor properly. See Splitting Elements.
Split the rail line as shown in the picture.
Finished result of extending railing