All trams are the same when coming to number of doors worldwide. As for 6 riders/meter, what is this based on size wise since we are not all the same size?? Is this summer clothing weather or winter as it take up space as well some winter clothing is more bulky than others?? Are they carrying things? What about Strollers, walkers, bikes and luggage as well the accessible person?? That number is too high.
Is the floor spacing based on Japanese and Chinese been shoved onto vehicles as toothpick as the current floor spacing is very flaw today???
All 5-9 modules unites have the same size door. Have said that, there are a few systems that run 3-7 modules that are about the same lengths as they are using a larger radius with 1-2 of them being shorter with no doors I recall and 2 in the others. then there is Croydon UK trams that have 4 double doors and one single doors on both side of the cars with the driver section reduce to have a single and double door beside the driver for a 3 unite 32m car.
Every subway train I been on that is old or new in my travels have 3-4 doors per car base on the length of the car. If you want to add more doors per car, how many seats will you see remove to allow more doors to be added to a car??? I guess riders will have to stand the full trip with no seat on board?? Been on a few subway trains where haft the seats on the end car behind the driver have been installed at all.
This is Croydon UK: London TfL 2546 (1998–1999) Bombardier CR4000 3 Section LRV with 5 doors per side at 30.1m with full driver cab and 100% low floor shot 2012