Pick this one up, unless you don't like micromanagement.
If you didn't preorder the game or haven't bought one of the few other packs that has a metro train in it, you only have a single metro train to choose from. Statistically, literally the worst one in the entire game.
This pack adds five fully modular trains to the purchasing list. While the amount of configuration you can do is limited (You're forced to have a power car for every pair of trailer cars, including the cab), the total length you can set (and therefore capacity) is far longer (and higher) than any of the 'Standalone' metros can offer.
It even gives you the ability to run short 2 and 3-Car trains that cost comparably little to run and can be easily extended as demand increases.
However, this pack also adds an element of micromanagement to the game. The custom comfiguration is never the default and you need to re-set the car configuration for every timetable period (Unless you clone them from another line). The sheer numbers also get hard to handle after a while, as you frequently forget how many of a particular car you have in a depot. Ten 12-Car trains (The maximum length) means that your depot has 120 individual cars that you have to purchase and manage individually. This particularly becomes a headache when it comes time to move a depot, you've got to sell or move them all individually.
I still say to get this pack, the number of new trains it adds is more than every other available pack combined and their utility is second to none, but there's a tier of micromangement that comes along with it.