You might try star jasmine. Smells heavenly when it blooms,does not grow out of control, and is a pretty green all year. It would have to be tied around the railing- it does not cling. It will grow in part shade or in sun. You may need to buy a bigger pot, though- I recommend a half wine barrel.
Renee

Thank you very much for your suggestion, Renee. I'll look into that - it sounds quite perfect for my purposes :-)

Renee's suggestion is good, and it will climb once started. Solanum jasminoides or Potato Vine is another that would work well for you. Al

I thought potato vine was an annual? Maybe I'm confusing it with something else, LOL.

I grow both star jasmine and solanum jasminoides. Of the two I prefer the solanum. Star jasmine makes a great modest-height groundcover, though. It doesn't cling, but it does twine a bit and then most of it sprawls outwards. I think of it as similar to ivy - it will climb, but it's just as happy to creep over the ground and smother everything beneath it.

Solanum also twines instead of clinging, but it has the great advantage of the stems remaining thin and flexible. It retains an airier, lacy appearance whereas star jasmine is a bigger, heavier vine in both appearance and actual weight.

I would be VERY careful about covering a stairway railing in any vine. I've done it - it's a lovely look - but if this is a railing that anyone really needs for stabilizing themselves so they don't risk a fall, you cannot adequately grasp the top of a railing when it's covered in vines. Bees especially love star jasmine, so you really don't want to be grabbing at a railing covered in it.

Both vines grow very well in the Bay Area and are fairly drought-resistant even in pots. Both take pruning, even extensive pruning, without a problem. Note that the variegated solanum jasminoides is much less vigorous than its green-leafed cousin - positively wimpy in comparison, in fact.