Some Coaster Challenges will require a Train Modifier or a Station Modifier.
The Station Modifier is a skin that can be added to your roller coaster station. It will boost the Coin storage of your station.
The Train Modifier is a skin that can be added to your roller coaster, like the station, but this one will boost the Max Peep Load of your ride, resulting in increased profit.
Right now, both are only available during special seasonal. You can get them in free and daily packs, rewards pack, and in any pack in the shop. They have a greater chance to drop in Coaster Card packs and Seasonal packs.
To assign them, enter the Coaster Edit mode, and tap on the Train or the Station icon of the Tool panel. Then simply drag the card you want to assign in the park.