。
The train came to a smooth stop.
Literal
Train [topic-は] smooth-ly [adverbial-に] stopped.
滑らかに ('smoothly') is the adverbial form of the な-adjective 滑らか, literally 'sliding-like.' Derived from the same root as 滑る (to slide) but used metaphorically for smoothness of any process. 停止 ('stopping, halt') is the formal Sino-Japanese noun for a stop, specifically for machinery and vehicles.