noun
extra service; relief service; interim service
Refers to an additional or temporary transport service (bus, train, flight, etc.) scheduled to handle increased demand, special events, or disruptions.
花火大会のため、臨時便が運行されます。
Extra services will be operated for the fireworks festival.
台風で運休になった路線に、バスの臨時便が出た。
A relief bus service was provided on the line suspended due to the typhoon.
増発 means increasing the number of services (e.g., trains, buses) on a regular schedule, whereas 臨時便 implies a temporary, often one-off, extra service.
Compound of 臨時 (temporary, extraordinary) and 便 (service, transport). The reading is standard on'yomi.