The Rhine-Main Transport Association (RMV) is the operator of one of Germany's largest public transport networks, with public transportation being the most important means of transport in Frankfurt.
www.rmv.de

The Verkehrsgesellschaft Frankfurt am Main - or Frankfurt Transport Authority (VGF) - is Frankfurt's main public transport operator. Over 350 rail vehicles provide comprehensive mobility on nine Metro and ten tram lines. The VGF is responsible for the network's infrastructure, security, cleanliness, maintenance and customer service.
www.vgf-ffm.de

Park & Ride (P+R) car parks
You’ll find P+R car parks at the various access roads leading into Frankfurt. Drivers may park their cars there and then take advantage of the bus and commuter train services to reach downtown Frankfurt quickly and easily – ideal for travellers wanting to get to their downtown destination directly and with a minimum of hassle.
www.pundr.hessen.de (in German)