This route was one of the more historical highways in the US highway system.  Even though it did not follow a famous trail nor path used by a military commander, it did traverse the battlefield of one of the most pivotal battles in the American Civil War.  From its western terminus at Lincoln Square in downtown Gettysburg to Blacksmith Shop Road in Cumberland Township, it passed through the historic borough as well as the southeastern section of Gettysburg National Military Park.  In fact, its intersection with Business US 15 was located near the Gettysburg National Cemetery, where the fallen Union soldiers are buried.

Not much changed along the route during its existence, except for one instance.  In 1958, the highway was widened from Whitehall Road at the northwestern end of Littlestown through to Kindig Road at the southeastern end of the borough.

Rumblings that Maryland wanted to decommission US 140 began to be felt in 1977 when the state sent the Borough of Gettysburg a notice they were planning to change the designation, and suggested Pennsylvania might want to change its designation to conform.  At their June 13, 1977 board of directors meeting, the Gettysburg Chamber of Commerce was asked its opinion of the proposed change by the borough council.  The directors notified council that they agreed that the road should only have one route number, rather than confusing drivers by changing the numbers at the state line.  However, they learned that there was one flaw in changing from US 140 to PA 97 to correspond with the planned MD 97 designation on the other side of the Mason-Dixon Line:  PA 97 was already being used in Erie County!  Chamber officials urged the borough council to support PennDOT efforts to persuade District 1 to give its PA 97 a new designation and move it to Adams County to replace US 140.  In the end, PA 97 would become a duplicated route designation posted in both counties.

The Maryland State Highway Administration made their intentions official on September 6, 1977 when they applied to the American Association of State Highway and Transportation Officials to decommission the route, with Pennsylvania's companion application following on September 22.  While both states cited a policy that was established by the then American Association of State Highway Officials effective January 1, 1959 which stated that any US route less than 300 miles in length and/or within one state should be eliminated as a reason, it was ironically Pennsylvania adding that the change would provide more continuous routes in Maryland.  With completion of the Northwest Freeway from Reiserstown to the Baltimore Beltway, MSHA saw no need for a parallel US designation and planned to change US 140 to MD 140 from Baltimore to Westminister.  From that point, MD 97 would swap alignments with the US route:  the new MD 140 would continue west to Emmitsburg and the Pennsylvania state line, while MD 97 would continue north to the state line as well.  PennDOT added in their application that a change to a state route designation would not take place until January 1979 to allow both states to release updated state highway maps and give local businesses plenty of time to address the change.  AASHTO's US Route Numbering Committee approved the elimination of US 140 at their meeting on October 28, 1977 with the Executive Committee approving the action during their meeting the following day.

Map showing the route of US 140 in Gettysburg from 1978 Map showing the route of US 140 in Pennsylvania from 1978
The route of US 140 in Gettysburg (left) and through the state (right) as shown on the 1978 official state road map, which is the last one printed with the route shown.  (Pennsylvania Department of Transportation)

Not everyone was happy about a US route being replaced with a state route.  The Borough of Littlestown disputed the change and filed a complaint with PennDOT through State Representative Kenneth J. Cole, but never received any follow-up from the Department of Transportation.

The designation was officially decommissioned effective New Year's Day 1979 with work to change signage taking place over the following weeks.  Richard Hackman, PennDOT District 8 traffic engineer, estimated the change would cost the state $2,500 mainly for sign replacements.

To date, this is the last US route to have been either partially or entirely decommissioned within the Commonwealth of Pennsylvania.

Western Terminus: US 30/PA 116/Business US 15 at Lincoln Square in Gettysburg
Eastern Entrance: Maryland state line two miles south of Littlestown
Length: 12.30 miles
Names: Baltimore Street, Baltimore Pike, North Queen Street, and South Queen Street
LR Designation: 42
County: Adams
Multiplexed Route: Business US 15:  US 30 to Steinwehr Avenue in Gettysburg
Expressway: None
Former Designations: PA 24 (1927 - 1928):  US 30 to Steinwehr Avenue
PA 31 (1927 - 1928):  Business US 15 to the Maryland state line
Decommissioned: 1979
Replaced By: PA 97:  Business US 15 to the Maryland state line
Links: PA 97
End of Historic US Highway 140 - Dale Sanderson