The following caveats relating to these shapefiles have been published by Network Rail. The original Microsoft Word document has been copied in to this file to make it more accessible. The original document was labelled OFFICIAL.
The following text is taken verbatim from the source:
The reference line is a single line to represent each ELR, this is an imaginary line which has been generated by creating a best fit against the main lines for each ELR. The dataset was originally generated from the CNM geographic measure line, which was created in 2005. The CNM geographic measure line was created from the CNM geographic route level dataset to be used for linear referencing purposes. The data extends to the full extent of the track TRID to ensure that all assets attributed to that ELR can be successfully referenced. The reference line is represented as polyline. This dataset being updated as part of the data improvement process, this is to ensure that this dataset directly relates to the NetworkLinks dataset.
The NetworkLinks dataset contains a geographic representation of the Network Model at track level ELR and TRID track centre line. This model is constructed of individual links that form the national network. The track level model is represented as polyline. The baseline data for this layer was originally created by the Corporate Network Model CNM in 2005. This was then updated with new survey data from the Geospatial Model Stabilisation Project GMS. Where GMS updates have been undertaken the track model has a positional accuracy +/- 0.5 meters x and y. Since the GMS project finished in early 2014 the data is being checked against GEOGIS, as part of the data improvement process and where necessary updates are taking place. Since late 2017 this updating process has now become part of managing INM and the Network Model and INM are one of the same.