Another way of looking at it: Every single part of a game must be calculated and/or randomly or procedurally generated in some way. Trying to figure out how to make passengers or factories pay differing amounts and still have things be possibly profitable and logical would be.... massively insanely difficult. What mechanism would you use?
Except that passengers and factories already DO pay differing amounts due to the speedbonus. Which is quite difficult to really understand, but thankfully, you can play the game without really understanding it for the longest time, since what it does mimics reality. You may not exactly know why your own lines suddenly don't make profit anymore, but you can relate to reality and think of upgrading them.
So you could have a mechanism like this:
1.) Give vehicles that can transport pax a comfort level. As a basic implementation, that's just a variable assigned to a vehicle. A more advanced implementation might include vehicles that can increase or decrease the comfort level of the whole convoi. (eg. a steam locomotive might decrease since the steam blocks the view/windows can't be open. A bord bistro might increase, etc.)
2.) Increase the payout with the comfort level, the higher the comfort level, the higher the transport fee. Of course, comfortable cars are also more expensive.
3.) Allow a configuration file to establish pax subclasses and their chance in each year. Whenever a pax spawns, it will be of one of the subclasses, decided by chance.
4.) Allow the same configuration file to establish minimum and maximum comfort level each class is willing to use. This can change over time as well (probably increasing)
5.) Do routing for each subclass, while only using connections with vehicles of an appropriate comfort level. This should cause as much additional performance demand as introducing additional good categories does, or like starting a mail network. So while there can't be an infinite number of subclasses, a few shouldn't have too large an impact.
6.) Pax will only enter vehicles of appropriate comfort level. This is to avoid using just one appropriate vehicle to unlock large parts of the map for all subclasses. It also means that if two different means of transportation connect the same two stations, some pax might only choose one, while some pax will only choose the other.
7.) THIS COULD ALL BE HIDDEN FROM THE PLAYER. As in, the game would continue to only display "Pax" without giving any indication what subclass each Pax belongs to. If you would explain it, it's just a bunch of mechanics. If you don't explain it, it's as simple as "If you have a higher comfort level, people pay you more to travel with you, but not everyone will afford it.", and casual players might not be confronted with how it works at all. Power Players can get the general information about which subclasses exist and which comfort levels they accept in a seperate window, much in the way how the speedbonus is only visible in the goods window and does not distract beginners.
EDIT:
The basic rate to transport a ton of something the same distance is the same in 1900 and 2000. The speed bonus means that you have to use a faster vehicle to achieve basic rate, but that has more to do with expected quality of service than inflation.
That's how it is done, but if it was displayed differently, you would have a decrease of the base rate over time, requiring you to offer more (faster service) to get the same payment.