A Private Number is a number, obtained over the telephone or telegraph from the station master of the station granting Line Clear or requesting points to be set or signals to be pulled off. This number is noted on the paper forms such as the Line Clear Ticket or Conditional Line Clear Certificate and can be verified later at the receiving station (the one granting Line Clear) or requesting the signal or points change. This is an additional safety device.
Private numbers are printed or typed up in advance on booklets which are supposed to remain in the custody of the station master or his immediate staff. In theory, it is not possible for anyone outside the station master's office to predict the next private number that will be issued since they are pseudo-random in nature and do not follow any sequence or pattern. Hence, the verification of the private number provides a good confirmation that the action it refers to was performed correctly and not in an unauthorized manner. Under rare circumstances, two consecutive private numbers may turn out to be the same or nearly so; in this case the second one is cancelled and a new one issued by the station master.
In addition to blocking or clearing trains, private numbers can be used to confirm control messages for rerouting trains, permitting unusual movements such trains on the wrong line, issuing new speed limits directly through the control office, or exchanging any other messages between section controllers and station masters. For instance, notifications of temporary speed restrictions, temporary line blocks orpower blocks, etc. Another case is that of closing the gates for road traffic at non interlocked level crossings (in this last case, private numbers are generally used only when the level crossing is on a block section and not within station limits).
Security buffs will note that while the possession of a valid private number shows that the action performed by someone was authorized, it is not an entirely foolproof system. In particular, the system does not guard against impersonation (the person to whom the private number is revealed may not be who he says he is, or the person providing the private number may not be the one authorized), nor does it provide non-repudiation (the person who is given the private number can disown having obtained it) or spoofing (there is no way for the person who receives a number to verify that the number provided is a legitimate private number before acting upon it). Hence, mechanical or other interlocking systems are still used in conjunction with private numbers.