I think more recently most Class I railroads in the US use speedboards similar to that which BN mentioned, although it would not surprise me if some, especially Class II or lower do not at all and rely on the engineers to know the territory, as they are tested to do. I have never worked for one so I don't know for sure.
Back in the day it was more varied from railroad to railroad, so in reality an answer would depend on what era, region, or even railroad you are looking at. Keep in mind that from what I've read of TT's, there was a MAS for the entire (sub)division and then occasional areas where this was reduced, either permenantly or temporarily.
As an example, the New York Central used those rectangular boards at an angle as mentioned by BN as the warning sign, placed ahead of the restricted area as determined by the braking profile of the track, and then simply a Yellow circle to indicate the beginning of the actual speed restriction. There would then simply be a green circle at the end of the speed restriction indicating maximum TT speed only, without typically stating what this speed is. The same applied for temporary restrictions, except the advance sign was square, the restriction sign was a yellow diamond with an "S" and the return to MAS was a green square with a "R".
http://www.multimodalways.org/docs/...C/NYC Standard Plans/NYC EC J-4 1-17-1963.pdf
http://www.multimodalways.org/docs/...lans/NYC Temporary Speed Boards 5-23-1956.pdf
For the record, MMW is a great resource for many US railroads, primarily Northeastern, but it contains a variety of information for a number of them - track charts, timetables, ETTs, plans, yard diagrams, etc. I've been known to spend hours on there just reading.