Perhaps JGR could comment on the possibility of adding speed restrictions via the advanced Signal routing feature
I hath been summoned...
In one word: no.
In more than one word:
It is unlikely that any mechanism to reduce the maximum line speed on a given track tile will ever be implemented. This is due to various factors. Firstly there is little remaining landscape storage space remaining. Secondly, due to the nature of the train movement code, it would seem to me that it would be necessary for each tile within a reduced speed zone to be marked with the required speed. This results in a UI problem of marking each tile. Marking only a single tile would result in the train accelerating back to normal after passing the tile unless some kind of distance-limited or toggleable state was applied to the vehicle, which opens up a whole new water-heating-device of scaled-marine-wildlife, with loomingly overbearing and evil consequences and side effects to trip up coders and users alike. Furthermore, unless you can find somebody with the necessary inclination and abilities necessary to undertake such an endeavour the odds of any attempt at an implementation occurring are less than satisfying.
If the NMA was implemented, some of these issues would cease to be of concern, however, I am less than absolutely convinced that the implementation of that (rather extensive and internally drastic) system will be complete in the sort of timescales which would be convenient for you. Do not also forget that in TTD train-movement lookahead and braking distance is clamped to half a tile, which kind of makes a mockery of the concept of speed restricted track segments and similar proposals, and from a gameplay POV makes them entirely unnecessary (barring the level-crossing fiasco, which Dale has addressed).
I hope that that has cleared up that uncertainty, bearing in mind that everything above is an unfounded opinion.