If you have a scanner, you may have heard the RTC (Rail Traffic Controller) give an "OCS Clearance" to a train. It might sound something like this:
"Item 1. OCS clearance #1234 to train CN 532, engine 2238. Item 5, proceed from Sparks to Andrews. Item 7, protect against Foreman Smith between Sparks and Andrews. Item 12, complete at 1225 RTC JBM."
What does all that mean?
First of all, OCS stands for Occupancy Control System in Canada. This is what is known as "dark territory" where there are no signals to govern train movement. All movements must be authorized by the dispatcher / RTC. This OCS clearance is the authorization for train 532 to occupy track on the Letellier subdivision.
You can read about OCS in the Canadian Railway Operating Rules, specifically the 300 series of rules.
In order to interpret the OCS clearance, you need to have a copy of CN's OCS clearance form (to know what the items mean) and a copy of the timetable for the appropriate subdivision (to know where the locations are).
Here's a blank OCS clearance form.
Here is the CN Letellier subdivision, from the August 1 2009 timetable.
OK, now we'll go through each item.
"Item 1. OCS clearance #1234 to train CN 532, engine 2238."
This is pretty straightforward. It identifies the clearance number for reference, and most importantly identifies what movement will own the clearance.
"Item 5, proceed from Sparks to Andrews."
There are three key items here. The two obvious ones are the bounds of authority, the timetable locations Sparks (mile 3.0) and Andrews (mile 36.2). There are no sidings listed on the Letellier subdivision, so the limits are the station boards at those two locations. If there were sidings, the limits would be just outside the fouling limits of the sidings (CROR rule 304.1).
The third key item is the word "proceed". This means that the train must go in the direction specified, and cannot back up without getting a new clearance. There are very limited exceptions, specified in CROR rule 308.1.
The RTC could have said "Item 6, work between Sparks and Andrews" instead and that would authorize as many backup moves as the train crew wished.
"Item 7, protect against Foreman Smith between Sparks and Andrews."
This tells the train crew that Foreman Smith is somewhere out there, and it is the train crew's responsibility to contact Foreman Smith and get permission to pass through. Foreman Smith may not even be aware of the train until s/he gets contacted.
"Item 12, complete at 1225 RTC JBM."
This shows the OCS clearance is complete and there are no further items.
Once the crew writes all of this down, they repeat it back to the RTC to confirm that they received it correctly. The OCS clearance does not go into effect until the train crew repeats the entire clearance including the last line.
That's enough for now. There are a lot of interesting variations possible but this gives the gist of the OCS clearance system.
EDIT: Added timetable page for CN Letellier subdivision.