is written in its own programming language, TOPSTRAN, which means no-one will dare (currently) to re-write it: that and just about every train related system uses TOPS data and it'd be a nightmare to change now!
wouldn't be that hard to replace, at the end of the day it's basically a database, and by modern standards a very simple database (stores only numbers, dates and text). Furthermore, the difficult bits of development like adopting a standard numbering system were done long ago & the simple integer based numbering would be relatively easy to use as the index for a new database.
I suspect the complexity of modern fault diagnostics has long superseded TOPS for modern stock, and that alternatives are being used by TOCS, ROSCOs and depots already albeit not in a standardised manner.
migration to a more modern system would allow multitude of new features (ability to store graphics, CAD drawings, etc, etc). but could include the same fields as the old tops, and have a NEWTOPSLITE interface which would be accessible using existing client software.