。
Train compartments quickly feel cramped.
Literal
Train [of-の] compartment [topic-は] soon becomes-cramped.
コンパートメント ('compartment') is a loanword for the enclosed train-seat booths common in European trains. すぐ ('soon, quickly') + 窮屈になる ('become cramped, feel confined') describes the claustrophobic feeling. 窮屈 is a な-adjective; the ~になる change-of-state pattern marks the transition.