Details. Tickets. for Hop On-Hop Off City Tours are bookable online, at the Tourist Information Offices Römer and Main Train Station. Tickets are also available from the event organiser and from the bus driver directly. Gray Line Frankfurt Sightseeing GmbH. Phone +49 (0) 69/40 05 02 01 0. info@frankfurt-sightseeing.com. 3nw9.