How the interlocking plant - and the operator - assures safety

Before clearing a path for a train, the interlocking plant checks that no involved track sections are reserved for another train. In a small number of well-defined irregular situations, operators can no longer rely on the interlocking plant, which prevents unsafe train movements. In such a case, the operator must fill out a checklist and enter a special command in the computer system.

In clearing a path for a train, the interlocking plant:

1. checks that all involved track sections are neither occupied nor reserved by another train,
2. sets the turnouts,
3. sets up the through path as a series of paths between two adjacent track sections, as might be set for switching moves governed by dwarf signals, and
4. sets the main signal to "clear".

Dealing with irregular situations where the interlocking plant cannot be relied on

In a small number of well-defined irregular situations, operators can no longer rely on the automatic safety design of an interlocking plant, which prevents the operator from authorizing potentially unsafe train movements. Some examples are allowing one or more trains to enter a track section:

• previously taken out of service,

• which the interlocking shows as occupied,

• in which the interlocking shows - correctly or incorrectly - road crossing gates as not down,

• protected by a signal that shows red or is not working.

In any of these cases, which require what is called emergency operation (Notbedienung), the operator must fill out a checklist before overriding the automatic safety features of the interlocking plant.

For example, after going through the checklist to return a track to service after it has been out of service for maintenance, the operator must "physically" erase the text "track out of service" (Gleis gesperrt) on a special subscreen of the computer system.