Ahh no! they've replaced the character-laden rickety old wooden bridge across the Cass River with this clean and functional item. DCs 4513 + 4761 head west with a decent off-season train of seven cars and two vans - the middle one is an open-air observation car with HEP in the middle, the rear is primarily for luggage with a small viewing platform at the train-end. As the weather can be great or miserable all year round (and often all of that several times a day) I'd pass on the longer summer days and see some snow on the mountains if I were you.
Trains on KiwiRail network from rail ferry terminal at Picton to Bluff. The Southern Alps are overcome at Arthurs Pass by the Otira Tunnel on 1 in 33 (3%) gradient. The longest rail bridge in NZ over the Rakaia River is near Christchurch.