These days, it's mostly either:
1. Overhead power line failure (which requires replacing half a mile of power line each time because they cut corners and made each section of cable that long) due to theft of the cable or vandalism of the cable
2. Overhead power line failure due to wind/storm/rain etc damage
3. Overhead power line failure because its shoddy quality; or
4. Signal failure (because the signals are ancient)
£30 billion+ on electrification, and the line is down atleast once every week between Reading and Paddington for the power line and once every week for the signals.... (never mind the rest of the line...)
/rant over
FH4 is sooo unrealistic because the Flying Scotsman runs 24/7 without any delays