Logical Data Model

One of the objectives of this project is to specify a general model of railway networks and their operation, as the first step in supporting the development of appropriate simulation and optimisation tools.

Network and signalling

The first component of the model describes the fixed infrastructure of the railway. This consists principally of the trackwork (the "permanent way"), together with the signals and routes used to control the passage of trains.

The definition of a modest number of entities and their associated attributes is sufficient to completely describe the operating environment and define even the most complex layout. These may be summarised as follows:-

The Network

The Network can be defined as a finite simple graph, which like any graph, comprises a set of nodes which are joined by a set of edges. Here an edge will be called a Track and the term Node will be used as in graph theory. The order of a Node is the number of Tracks connected to it.

• Track - a section of railway line connecting 2 Nodes
• Node - a point at which a Track section ends, and may be of order 1, 2, 3 or 4.
  The following Node subtypes are defined:
• Line_limit - a Node of order 1, the end of a Track section which is not connected to any other Track section. There are 2 subtypes of Line_limit:
• Buffer stops - where a line ends, e.g. in a dead-end platform or siding.
• Fringe – at the limits of the area which is being modelled. The line does not in fact end here, but this Line_limit node defines the end of our interest in it.
• Track_join, which may be further subtyped:-
• A join between 2 Track sections, which occurs where it is required to split a section of plain track into separate sections with different attributes, e.g. a maximum speed or belonging to a different track circuit (see below).
• A join between 3 Track sections, representing one end of a set of points, where trains may deviate from one line to another.
• A join between 4 Track sections, the simplest case being where one line crosses another.

Figure 1: Subtypes of the Node entity.

A third subtype of the Node entity is defined in the next section.

Some additional entities are required to complete our definition of the layout:-

• Location – an entity giving a specific identity to a certain part of the model area, e.g. a station or named junction.
• Line – an entity giving a specific identity to a grouping of any number of Track sections, e.g. the “Up Fast line”. Some instances of Line will “end” at a Line_limit node of the Fringe subtype, and represent running lines on which trains may be timetabled to enter or leave the model area.
• Track_cct – one or more Track sections which are grouped together for the purposes of detecting the presence or absence of a train.

Signalling

Superimposed on the network infrastructure is the signalling infrastructure.

Firstly we define an additional subtype of the Node entity, Signal. This is defined as a Node since it is always necessary to distinguish between the two Track sections either side of a signal.

Three additional entities are needed:-

• Route – this can be regarded as the “basic unit” of signalling, i.e. of rail traffic control.
• Each Route runs from one Route_end to another; a Route_end can be either a Signal or a Line_limit. Each Route_end can be the start of several routes and also the end of several routes. There may also be more than one route between the same two Route_ends.
• A Subroute is an instance of a Track section being used by a Route. In other words, a Route comprises a number of Subroutes, each of which corresponds to one Track section.

As noted above, instances of the Track_join subtype of the Node entity may be defined simply to divide track into smaller sections with different attributes. Particularly relevant to the signalling system is the case where the Track_join separates different Track_ccts.

The relationship between the entities defining the network and signalling is summarised in Figure 2.

Figure 2: Logical Data Model: Network and Signalling

Timetable and trains

Having defined the network and signalling, we can now define a few additional entities to contain the specification of the trains to be run within this infrastructure.

• Timetable - the collection of data describing the required operations on a particular day in our defined area.
• Train_type - identifying the different types of trains which can be run and holding the attributes common to trains of a specific Train_type.
• Wtt_train - a specific train which is to be run.
• Wtrn_loc - the timetabled "appearance" of a train at a specific Location.
• Train_term - the termination of a train at an instance of Wtrn_loc.
• Plat_type - an associative entity denoting permission for a Train_type to stop at a specific Platform.

The relationship between these entities and those defined earlier are summarised in Figure 3.

Figure 3: Logical Data Model:Timetable and trains