The PATrain, also known as the Mon Valley Commuter Rail, is a defunct commuter rail service formerly operated by the Port Authority of Allegheny County in the Monongahela Valley in the US state of Pennsylvania. Service began in 1975 when the Port Authority assumed control of the Pittsburgh—McKeesport—Versailles commuter trains operated by the Baltimore and Ohio Railroad (B&O) (part of the Chessie System). The Port Authority discontinued the service in 1989.

In the early 1970s the Port Authority (PAT), which since 1964 had controlled all bus and streetcar service in Allegheny County, had negotiated with the B&O and Pittsburgh and Lake Erie Railroad (P&LE), the last two private sector commuter operators in the region, about the possibility of expanded rail service. At the time the B&O operated six weekday round trips from Pittsburgh to Versailles, while the P&LE operated a single weekday round trip from Pittsburgh to Beaver Falls. The Pennsylvania Railroad had ended service on its six commuter routes in 1964, citing lack of patronage.

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Neither the B&O nor the P&LE showed much interest in expanded service, citing existing operating losses and declining patronage. PAT then proposed that it take ownership of the B&O's service, with the B&O operating it under contract. The B&O trains made the run from McKeesport to Pittsburgh in 25 minutes, twice as fast as comparable bus service. Among the strongest supporters in the local government were then-mayor Peter F. Flaherty and County Commissioner William Hunt. Another champion was Harold Geissenheimer, PAT's director of transit operations. In 1974 PAT estimated capital costs for a three-year trial at $1.7 million, plus $1.9 million in operating costs. The capital costs would be split between the federal government, the state of Pennsylvania, and the county, and would include the purchase of two locomotives and nine coaches. PAT would increase the existing two-car trains to four-car trains, while also increasing frequency of service. The per-passenger subsidy was estimated at 95 cents, compared to between 6 and 13 cents for the typical bus passenger. Daily ridership then stood at 300; PAT's best case projection was 3,000. A proposal by Hunt to extend service further to Elizabeth was unsuccessful: the route was owned by the P&LE, which requested $500,000 to rehabilitate the line. This the state would not provide, as the line would still mostly carry P&LE freight traffic and not commuters.

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