For this example you will want to start out with a Std Base and take it to the Assembly Editor, in the Section View.
1. Select the Bottom Rail and change the width to the necessary size.
2. Next, go to the Face Orthographic View, right click on the Bottom Rail, select Edit Shape and shape the rail as necessary.
3. Now, you want to go back to the Section View. Go to Properties and select Toe.
4. If you want the underneath of the assembly to be open all the way back to the wall it is attached to, type in 0 in the Toe Height box and hit OK. The result should look like the following:
Or, if you want a sub toe to set in behind the arched rail, type in 0.001 (if you are using metric - 0.1 will suffice) in the Toe Height box and hit OK. The result should look like the following: