I am not in any way excusing either the failure of the UK government to invest in Wales' railways, or the Welsh government's management of the franchising, or ATW's operational effectiveness BUT I read recently that there are no spare diesel-electric units/carriages in the UK.
This is from ATW's website, albeit an FAQ written a few years back:
"As a result of the ever increasing popularity of our services (more than 1 million more journeys every year for the last 11 years), we acknowledge that capacity is a major concern for our customers. The finite number of trains in our fleet acts as a significant constraint on our ability to add extra trains or carriages. We have investigated options of obtaining additional trains, but due to continuing passenger growth across the entire UK rail network, all Train Operators are holding onto trains and there are no suitable trains to lease at this time. The rolling stock (train) owners have also advised us that there unlikely to be more diesel trains to lease until sometime in 2017 or 2018 when the national electrification programme should see some trains become available. In the shorter term investment has been made to enable us to provide a locomotive and 4 carriages to operate services between Holyhead, Chester and Manchester, meaning 11 of our busiest services will have more capacity from December 2014 with 2,450 extra seats per week."
https://www.arrivatrainswales.co.uk/FAQ ... rriage-on/