I would like to add to the mix of the consersation that desert will always take slightly longer than temperate due to oil and goods being the better of the cargos, whereas on temperate you have a lot more choice and can do several lines using a combination of coal, farm and goods. But to the point, on desert 150 mil using diesel trains it is possible for a reliable 8 year win if going for oil and goods and throwing a little bit of pax in as a potential start depending if there isnt a great oil well start.
As for the long goal desert server we run, we did have it set to 2000 for awhile till it just got abused by people going for wood, achieving silly amounts of points for ridiciously quick wins. So to counter this the start year was placed back to 1975 however ALL city populations were increased significantly on map generation, therefore pax starts are highly profitable which therefore remove quite a number of years off your game. Players would complain that they preferred the 1975 over 2000 to a starting year as well.
So overall it is a balance of people wanting faster games because they feel like it is slow and boring and other people just wanting to spend an afternoon or evening on a server playing for fun.
With that said though, you can take a step back and look at it from another angle. There are four different climates that are playable which are temperate, desert, winter and toyland. Each climate has different gameplay elements and the speed at which you can achieve a goal value is vastly different. If you want the fastest possible win ever the order of climates actually stands as the following: Toyland, Temperate, Winter followed lastly by Desert. Which can be seen as quite weird because the toyland trains are actually slightly slower than that of the desert trains.
Toyland is the fastest because there are three "Goods" producing industries which are Sweets, Toys, and Fizzy drinks which are all very similar in its cargo payments, and the primary industries for these cargos are all high producing industries therefore any map generated on toyland will be inheritedly quick. My personal fastest time on a 25 million euro goal on toyland on a 512x256 map is 3 years and my slowest time 4 years. Compare the same to the temperate 25 million euro goal where yes, you can achieve 3 years but it is extremely difficult (connecting everything farm you can see as quick as humanly possible then going early goods) and fast paced, 4 years is the time that the best players thrive to achieve. However a lot of players do not like the toyland landscape at all. They mostly have prejustices about it or dislike the more intense colours or mostly just hate the trees (which are easily disabled via the server settings so none appear on map generation or you can change your transparency settings)
So in my honest opinion the way the trains were programmed is quite well thought out. Catering for basically all types of players personal tastes. On toyland you can have faster games with various "goods" producing industries with slow trains of around 150 km/h but if the climate puts you off you can play on temperate. On temperate you can go via the goods route or the coal route to win so this gives players the chance to decide how much work they want to put into their game to make lots of money. Also giving them the chance to choose faster trains to give a real sense of speed and break up the monoity of slow trains (160 km/h or less) till 2000. Desert has always been more of a relaxed climate, something you can play over the course of an afternoon, moving cargos about and trying to grow little towns on desert tiles to make them huge cities to generate more and more passengers. Not as simple as a temperate climate but offers more a challenge to players who are little bored playing the same old climate time and time again. Then we come to winter. It's perfectly placed between temperate and desert with the fact there is the famous wood -> paper -> goods line which offers players to go for a three tiered manufacturing line rather than just the one or two tiered lines. With the presence of coal on winter too this allows players to have good incomes with slower trains therefore taking a lot of so called "wasted years" out of your game time as you barely have to wait for long till you have enough money to build like crazy.
So as I have said its all personal preferences to which you play, and the reasoning for the train engine speeds could have been chosen to better suit these preferences.
Though the case will remain, the faster you can build, and the better you plan your routes (to give maximum income per tile) the faster you will always win regardless of the actual train speed. Because from either a 1990 or 1975 start, all the company value servers we run can and have been beaten before the release of the monorail engines (this includes the 1 billion desert server). While it is a fact that the game is inheritably quicker with monorail or maglev, the games we run shouldn't reach that ingame year if played optimally unless it was to start at 2000 or 2020. But playing optimally isn't always fun nor is it relaxing because as a game the top propority should always be enjoyment and taking pride that a network you build is starting to flourish.