Of course you're spending money, but there's a net reduction in capital expenses since you're freeing up higher capacity rollingstock for better use elsewhere, replacing it with cheaper, more appropriate vehicles.

Rather hilarious that you picked the Tyne and Wear Metro too. Compare the pair:

Rather hilarious you fail at basic maths, we have:

>Tyne & Wear Metro:

>>>39 tonne for a 56m train, scales to 117 tonne for 168m for three sets, or 0.7tonne/m

>Melbourne X'trapolis:

>>>six-car set at 244 tonne for 143m, or 1.7tonne/m

Which makes sense, since the listed capacities are:

>Tyne & Wear Metro:

>>>64 seats + 188 standees, or per three sets 192 seats (504 total), 1.1seats/m & 4.5pax/m

>Melbourne X'trapolis:

>>>listed capacity of 528 seats + 266 standees (crush 1394), 3.7seats/m & 9.7pax/m

So such stock is half the size of our trains, far more suitable for the capacity needs of Williamstown/Altona etc.