That puts the Z/ZA's at 660 tonnes each, somewhat more than they're rated at now. Maintenance issues? As for the 3500 series Cat, they failed dismally in fast ferry applications that the RK engines managed to fill. Caterpillar left the segment but are trying to break back in. Ten years of development may help. I was hoping the EMD 8-710 would be the pick. Commonality and all that. Too many accountants involved perhaps? But then, as Sargent Schutz said, i_know_nothing!
Well, not nothing, perhaps, but you are wrong about the ferries....
The less than successful fast ferry engines were the very much bigger Caterpillar 3600 series, which were equally unsuccessful in locomotive applications such as the MK 5000 type.
The 3500 has been used successfully in many locomotives, including many of the pre-genset switchers built by Motive Power. It is available for small ships and was actively considered for re-engining of the Fremantle class patrol boats (but at the expense of a reduction in power and speed compared to the MTU 538) but it is far too small for fast ferry applications.
I'd expect that it would be cheaper to buy than an 8-710ECO. The 3512 may not last as long, but the through life cost for normal accounting periods might be lower. I don't imagine they expect the locomotives to last 40 years, which EMD engines frequently attain.