If you want it to be simple and work, just get the 17 foot MFJ, and clamp it to the metal railing. If the railing is of a reasonable size and the sections are welded or screwed together for good connectivity, then if the total length is say 16 or more feet in each direction, or even something longer in one or more directions, it will probably work just fine since you can adjust the whip length for whole system resonance. The whip may end up 15, 16, 17, 18, 19 feet or something like that. No problem except there may be some SWR and some common mode current on the coax. Use a common mode choke and that solves one problem, and a tuner in the shack or rig(if it is even necessary) will solve the probably small SWR problem especially if your coax loss is only something like one or so dB. Those of us who operate portable QRP typically have fine results using a chain link fence, metal picnic table, etc. as our counterpoise. Is this ideal? NO, but is simple and I really doubt that you would lose more than a dB or two. The advantage is it would be easy to tune and get you on the air immediately. If the system is satisfactory, you can always replace the whip with something like a 10 foot piece of conduit with a fiberglass bicycle or snow marker on the top and use the whip for portable operation in a park if you do not think the whip will survive long term. If you can install 3 or 4 elevated radials each 1/4 wavelength long this would naturally be a bit better. Another advantage of the whip, when the upper bands open up in a very few years, simply shortening the whip for system resonance on say 17 or 15 meters would be a snap. Since we do not know if the railing is well grounded to the building, or has a resistive connection, it is impossible to predict what will happen. However I have found that perhaps 95 % of the time when I operate portable, adding any large metal object either makes no change or improves performance. Very seldom does this degrade performance....so with the limited info I have, lets say your odds of success should be good that will be a very useable antenna. If that antenna is a "keeper" you could keep that length and put a remote automatic tuner right at the base and use it on 20, 17, 15 and even possibly on 30 meters. If you want a shorter antenna, make it resonant on 17 meters, and the remote atu should tune it up just fine on 20, 17, 15 and possibly other bands. If you will only use the antenna on the band it is resonant on, then a tuner in the shack (if necessary) should be OK with only a little additional loss. If you want to use the antenna on a different band than it is adjusted for, then a tuner at the base of the vertical would be preferable. Happy experimenting Rick KL7CW PS I have never tried the MFJ whip