I did some preliminary work about 6 years ago on a group of railroad bridges that were planned for conversion to a hiking trail (the Rails to Trails program). At that time we could not locate any formal standards. There was a non-technical book that described some successful conversions, but I don't recall the title or author.  We did our preliminary designs using engineering judgement based on accepted standards, such as AISC. Lack of funding caused a dramatic reduction in scope of the project (the trail was established, but the bridges were not converted).

Trail bridges could be classified also as pedestrian or briddle path bridges. These should be designed for the pedestrian loads (AASHTO) and checked for occasional light vehicle (maintenance, emergencies, deliveries). The vehicle loads should be limited to the type of load which the bridge would likely support. The typical specification could be seen here:
http://www.continentalbridge.com/assets/pdf/spec.pdf

As a follow-up to Wiktor's post, look at the AASHTO Guide Spec for Pedestrian Bridges

I think the best information on timber trail bridges can be found through the US Forest service:

http://www.fs.fed.us/na/wit/

At minimum, you will probably want to look at the trail bridge catalog:

http://www.fs.fed.us/na/wit/WITPages/bri...

And their free publication on Timber Bridges: Design, Construction, Inspection and Maintenance:

http://spfnic.fs.fed.us/wit/WITpubs/Pubs...

Thanks for all the good input!