First of all, what is a 'walking dead'? Trains don't walk, so maybe let's use some terminology that makes sense instead of something that seems like a populist soundbite to get people riled up? "Old train" is fine, it's clear, and you don't sound like you have a personal conflict against it.
Second of all, and I'm going only by the perimeters of your own argument that you have posted here: if your fleet is in bad shape and you need to either rebuild it or replace it, with the caveat that rebuilding costs only half as much as a new vehicle, and therefore you can rebuild twice as many vehicles as you can outright replace, why on earth would you choose the replacement option? Your clients will not thank you if they suddenly find their trip comes half as often as it did before. But hey, at least we got some shiny new toys that we can show off, right? Obviously in the real world the decision of rebuild vs. replace is often more complex, and differs in every circumstance, and sometimes rebuilding isn't even feasible, but I'm just addressing the argument that you yourself have put forward that is somehow supposed to be an argument in favour of replacement.