。
Train travel is in decline.
Literal
Train [of-の] travel [topic-は] decrease tendency [in-にある].
~傾向にある ('to have a tendency toward, to be trending toward') is a formal analytical phrase common in news and reports. 減る is the intransitive 'to decrease,' and 減る傾向 ('decreasing tendency') becomes a noun phrase marked by にある to describe the current state. Typical of written Japanese in journalism and academic prose.