。
The train is running behind schedule.
Literal
Train [topic-は] scheduled-time [than-より] is-running-late.
定刻より marks 'compared to the scheduled time,' with より as the comparative particle. 遅れている is the ~ている form of 遅れる ('to be late, fall behind'), producing the resulting-state reading 'is (currently in a state of) being late' — not the momentary action of falling behind but the ongoing condition of lateness.