Streamlined makes use of the Dutch guilder (or 'gulden' in Dutch), the currency the Netherlands used since the 14th century until the euro took over in 2002. In a tycoon-style, gulden can be used to purchase better locomotives and wagons to earn more gulden. Additionally, experience is also used.
Acquisition of gulden
Playing
Gulden are earned by driving trains and completing routes. A more in-depth explanation is give in the rewards guide.
Purchasing
Gulden can also be purchased with robux through the in-game shop. This is the fastest way to get gulden.
Acquisition of experience
Experience is used to increase your level, which in turn will unlock routes and make rolling stock cheaper, up until their unlock levels. It can only be acquired by completing routes. The rewards guide provides an in-depth explanation on how its acquisition works.