It depends on how speed came to be 100 mph.
I would do this (warning: hacky):
1) Open Tracks.bin in Networks in your route folder, using RW-Tools. Or convert to .xml using serz.exe, then use Notepad++, and remember to use serz.exe to convert .xml to .bin again.
Search for the word trackrules. Most people keep their track rules in a folder with that name, so this is a quick proxy.
Note what you find. What matters is the stuff between > and < in the line you find plus Provider and Product a few lines above.
You should only find exactly one track rule used, in theory.
2) Open the track rule file, found in Asset/"Provider"/"Product"/"the path you found".
Search for DefaultSpeedLimit. Primary is for fast trains, Secondary for freight. Edit as desired.
If that still reads 70 and 50, you set the speed in World Editor. In that case, the safer way is to mark all track in World Editor and set the speed there. This can be tiresome.
The alternative is to search for Network-cTrackNetworkSpeedLimit in Tracks.xml (Tracks.bin) and delete the XML element wherever you find it. This means that you delete the line that starts
<Network-cTrackNetworkSpeedLimit ...
and the lines that follow, up to and including
</Network-cTrackNetworkSpeedLimit>
This can be tiring, too. What you consider safer is up to you.
Be sure to keep backups, if you follow any advice from me. I am dangerous and my advices even more so.