On CN's and CP's mainlines, the speed is controlled by speed limit signs. On CP, these are yellow triangular signs (about the size of a yield sign) with the speed limit listed in mph. This speed limit is also controlled by the signals on the line. Green-over-green means the train can go the speed limit. Yellow-over-green means reduce the speed to medium. Yellow-over-red means reduce the speed to slow and be prepared to stop at the next signal. Red-over-red means stop. You will probably note that most signals on the mainline are alongside passing sidings. The red-over-red can be flashing which means the train will take to the siding. The dwarf signal controls the train movement in the siding. Red means stop and green means the train can exit the siding.
In Ontario, a lot of the railway lines are operated by short-line railroads. Traffic on these lines are operated by what is referred to as OCS - On Company Service. This means that the train is the only train allowed on that line. The speed limit is usually controlled by the condition of the track - usually around 25 mph.
And to confuse matters even more, sections of track, whether mainline or branchline, can be subject to "bad orders". The train crew picks up their "train orders" before departure. Amongst other things, the train orders specify those sections of track (the mileage) where the train is to go at slower speeds.
Mileage markers are posted alongside the track and on the signal masts. Mile 0.0 starts (and ends) at a Division point. For example, Smiths Falls is Mile 0.0 on the Belleville sub (Smiths Falls to Toronto yard) but Mile 999.99 (I don't know the mileage) on the Winchester sub (Dorval to Smiths Falls).
When railfanning, a handy aid is the Canadian Trackside Guide published annually by Bytown Railway Society. It includes the mileage points for each subdivision. Using the Trackside Guide and looking for the mileage markers alongside the tracks, on the track signals, and on the crossing signals, you can get a whole lot of information about the track that you are looking at. The Trackside Guide also includes the length of each passing siding.
While, on the same section of track, you might see slower trains, it doesn't mean that those trains will be slow the next day. It all depends on a number of conditions.
Another speed limiting condition is the speed capacity of the switch. Some switches along CN's Belleville sub are capable of taking trains going at 50 - 60 mph. Others have a slower speed capacity. In other cases, for the trains to do the maximum speed limit, the sidings are "locked" into place and have to be manually "unbolted". For example, on VIA Rail's Smiths Falls sub, the switches on the passing sidings are locked - ie they can't be used. This allows the VIA trains to go at maximum speed. However, when VIA brings in track maintenance equipment, the switches are unlocked so that the ballast tampers and regulators can be stored on the siding. When this occurs, "slow orders" are issued and it take the VIA trains an extra 10 minutes to go from Ottawa to Smiths Falls.
Bob M.