"The DfT is also considering the cost of replacing or upgrading some on-train wifi equipment installed in the middle of the last decade, and whether passengers on shorter journeys use their own mobile phones or data rather than connecting to an operator’s wifi."

Well, based on my experience dealing with in-train coverage both working at a telco and as a traveler, if their hardware was deployed nearly ten years ago, then it is quite likely that their passengers _are_ using their own data plans instead of it.