All tracks are available before there is a train able to reach the speed, this sounds quite normal (on the contrary: having a train that has no tracks to run on would be lame). This does not alter the gameplay, so I don't see any reason to push it back. Maybe the point of this request is decreasing this track's cost and maintenance as later means more technological availability ?
Although I agree that it should be available before the trains that need it (to allow people to upgrade ahead of time), the problem is it is available decades before. For over 30 years it provides the user with nothing more than pointless choice that clutters up the UI because there are no convoys that need its 200 km/h speed limit that cannot use the 140 km/h or 160 km/h track. Having a lot of build options available is good, but not when they have no purpose.
Currently you might as well make the High-Speed 400 km/h track available from the start since it is as useful at the time and wastes even more money if used. By it not being available from the start means that way speeds are meant to rise with convoy speeds. To me it would seem the 200 km/h track being available so early was either a mistake or there were intended to be faster convoys earlier on.
At 1940, the intro year of 200 km/h track the fastest non-broken engine is the RVg "Flying Berliner" (1936) at 142 km/h which can still use the 160 km/h track introduced in 1930 (6 years from available to usable).
At 1950 the fastest engine is the RVg Hiawatha Express (1943) comes in at 151 km/h. Still uses the 160 km/h track.
At 1960 the fastest engine is the RVg 2-4-0 "Express" (1951) comes in at 152 km/h. Still uses the 160 km/h track.
At 1970 the fastest engine is the MJHN Deltic (1966) comes in at 160 km/h. Still uses the 160 km/h track.
At 1980 the fastest engine is the Haru Spike PSE (1978) comes in at 270 km/h. This uses High-Speed 280 km/h track introduced in 1970 (8 years from available to usable).
At 1980 there are also the Haru BR-103 (1971) and Haru HST (1976) coming in at 200 km/h. This uses the 200 km/h Express track introduced in 1940 (31 years from available to usable
For 31 years it will clutter your UI as a track choice before it is actually usable by any trains. To put it in perspective the track retires at 2000 which is ~29 years after the first trains that can take advantage of it. This means that the track spends more time on the UI not being usable than being usable.
I would recommend moving its intro year to 1970 (1 year from intro to usable) since that would give people some time to deploy and upgrade their networks in preparation for the new engines that can use it. If you feel that it does not give enough time then 1965 (6 years from intro to usable) as that is a similar to other rail types.
Same goes for VT98. Perhaps the real problem is RT-120 vs VT98 ? Increasing RT-120 capacity would definitely solve those two problems ... What do you think about it ?
Currently I find myself at 1955 using 2 types of passenger/mail trains. High-speed for main routes (such as to an exchange), and combinations of lower speed ~100-120 km/h engines for gathering passengers. I do only operate point-to-point lines however so maybe the higher capacity is useful for multi-stop lines. In any case the slower convoys probably need lower running cost and/or higher capacity since currently they are just not worth using (nothing stops you using faster convoys which cost about the same to run and earn a lot more).
When changing capacity remember to raise power since the extra weight slows the convoys down.
Another bug. The "Haru Plan V." Electric engine cannot ever reach its top speed of 130 km/h. It only manages 129 km/h unloaded and 115 km/h loaded.