It might be an unwritten agreement to avoid eye contact, an understanding that certain people will be given seats in preference to others or an assumption regarding what constitutes personal space and acceptable physical contact. Latvia has variations on all these themes, yet it seems to me the most typically Latvian aspect of using public transport is something else.
Put simply, it is this: preparing to get off the bus/tram/trolleybus.
The accepted routine of using public transport in most of the world runs something like this: wait at stop, climb aboard, buy ticket, sit down, wait for stop, get off at stop.
The Latvian variant runs thus: buy ticket in advance, wait at stop, climb aboard, sit down, stand up at stop before your stop, move gradually towards door during interim period between previous stop and your stop, establish clear protocol regarding order in which passengers will dismount on a first-come, first-serve basis, wait for stop, get off at stop within 0.5 seconds of doors opening, make mumbled comment about people letting other people off the bus before getting on themselves, resume life.
It is this understanding that you prepare meticulously and publicly to get off at the stop before the one you actually want that is distinctive. It takes some getting used to. On the positive side of things, it gives a brief frisson of excitement. All those people gathering around the doors - what can it mean? - there must be something pretty exciting at the next stop! It's as if the grannies who make up about 50 percent of any busload are racing greyhounds just waiting to spring from their traps in order to be first into one particular hairdresser's seat for a red tint.
On the other hand, it suggests a latent fear that unless everyone is prepared well in advance and establishes some sort of undeniable critical mass near the doors, the driver might not stop at all, or will teasingly open the doors for an instant before slamming them shut and roaring ahead with what are essentially now hostages.
As a result of all this, the unprepared (i.e. tourists) who assume they can simply leave their seats in a casual manner and get off when the doors open, will frequently find themselves unable to exit the bus/trolleybus/tram at all as they have left it too late, and are swept back into the vehicle by an onrushing tide of humanity who are storming aboard as if they have just climbed out of a trench under heavy bombardment.
This method of getting off buses makes no real sense but it does at least give a little low-level drama to any journey. And this practised familiarity with preparing well in advance, lining up at the door and then leaping out without pause suggests Latvia probably has excellent paratroopers.