At the start of May, the timetable suggested two buses required, but I just saw the one.
At the start of June, the City Sightseeing operation is Warsaw was using just one bus on Fri/Sat/Sun only
Also noted was the former Bullocks Olympian, but it isn't open top and seemed to ahve closed down.
In July, 2 operators were running
Stadtrundfahrt Dresden seemed to have 5 or 6 buses running, with HH running 2 or 3.
Various piccies in this lot
Bex (City Circle) - B-EX 4600, B-EX 4900, B-EX 5292
BVB (City Circle) - B-CC 5620, B-CC 6621, B-CC 7623, B-CC 8624, B-CM 834
Gullivers (City Circle) - B-DQ 1089
Gullivers - B-DC 1082, B-DG 1083, B-DI 1086 (at depot)
Der Tempelhofer - 108, 115, 117, 119
Big Bus - 9924/5/9/30 (in service), 9923/6/31/2 (at depot), 9927/8 (at depot de-registered)
Top Tour - B-RR 676, B-RR 1492, B-RR 1709
City Sightseeing - 9923-6/46-8
Stromma - 7004-7/10
Gullivers now appear to have joined City Circle. Stromma and Big Bus were operating a joint timetable and tickets.
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Photos from the Gulf Region - http://citybus.shutterfly.com/