Any ideas on what to do with old metal porch railings for the backyard

Upend them and use them as trellis for climbing roses/vines...can also be used to create privacy screen around a patio, again with climbing something(s) on it...or use them as a backdrop for funky yard art

Fence in a garden.

Hang them as hard art. Paint them a fun color and mount pots of flowers in them

that will depend on the size

These 'strainer flower' were in today's Home talk email. You could use them for the stems.

http://www.hometalk.com/search/posts?filter=strainer%20flowers

Have fun!

If space allotters anchor them in a garden and grow flowering vines on them based on your hardness zone

Weld some legs on, or have them welded, that are long enough to provide stability and voila, you have a trellis.

If you have a storage building in the backyard, they might make a cute fence on the side facing your yard. And you could hang a lot of pots of plants on ten feet of railing. If that's not a bright enough area, run the railing out from your back door or patio. The plants would be convenient and easy to move around. Best wishes.

Stand them up against a building or the house and hang potted plants on them!