There are currently five passenger classes, "very low," "low", "medium", "high" and "very high". I suggest they be simplified to "low", "medium" and "high". There would be two benefits from such a change. First, the path explorer would require 40% less disk space, memory and computational power. Second, it would simplify the player experience which is currently too granular. While in reality there may be a price difference between 3rd class passenger seats and say, trams or buses (although they are priced identically in my city), I don't think the difference is significant enough to warrant a separate class as air travel or 1st class passenger seats would — especially as it increases complexity and performance costs.
To illustrate the latter point, I surveyed all passenger rail lines on the bridgewater-brunel server. I found that the vast majority of "low" class seats were manually reassigned to "very low". I have tabled the results by company below:
Crandon and Lakes Railway
22942, 41% ("low" seats remaining as "low" or higher)
33582, 59% ("low" seats reassigned to "very low")
North Western Transport
396, 30%
936, 70%
Central National Transport
0, 0%
27831, 100%
Bay Transport Group
3796, 3%
127364, 97%
Ves Transportation
4514, 55%
3640, 45%
Far Eastern & Western Railways
3552, 3%
101910, 97%
Northside (my company)
0, 0%
18732, 100%
Arlspike Fisherman
4038, 14%
23908, 86%
Great Highland Railway
3049, 8%
34176, 92%
Total
42287, 10%
372079, 90%