The Northern Pacific started planning its Wahluke Branch in 1959. The purpose of the line across this area of the Columbia River Basin, known as the Wahluke Slope, was because it was to soon get irrigation water on about 125,000 acres from a new irrigation canal being built as part of the Columbia Basin Irrigation Project. According to Ken Cook, NP's agricultural director, this area would produce some of the highest yields of sugar beets, potatoes, dry beans, fruit, mint and seed crops, forage and grain ever recorded in the Pacific Northwest. The line was to be constructed from Mesa, on the NP main line about 21 miles east of Pasco, west to Mattawa, a distance of 54.7 miles. The original plan was for the line to leave the mainline at Connell instead of Mesa. No reason was given for the change in plans. The NP was planning on spending $6 million dollars to build the line.
Mattawa is located on the west shore of the Columbia River just a few miles south of Beverly, where the Milwaukee Road crossed the river. At this time, Mattawa had a population of less than 100.
Authorization to build a 55-mile branch line to serve the Wahluke Slope area in Grant County, Wash., was received by the Northern Pacific from the Interstate Commerce Commission on Oct. 5, 1967. The ICC denied an application from the Milwaukee Road, which had been competing with the Northern Pacific for rights to serve the Wahluke Slope area.
Tracklaying began in July of 1969. The first 31 miles of the line will be finished by year’s end. But later that year, tracklaying stopped. Profiles for the branch show the end of the prepared grade at MP 53.29, with a future end at MP 54.75. Track was never laid beyond MP 21.33, at the proposed station of Sagehill. The bridges were built to the end of the prepared grade and were all of heavy design. The rail used was heavy second-had 112 lb welded rail. The railroad left the line unfinished because business conditions changed and felt that there wasn’t enough agricultural development in the region to justify completion. The Burlington Northern held permission from the ICC to complete the branch as late as July 1978.
Beyond the mid 1970s, the line was not used beyond Basin City due to a slide in the vicinity of MP 11 or 12, though the rail was good beyond it. The line was removed between Sagehill and MP 11.20 in 1983, and some time later removed to MP 10.0, about 1.5 miles west of Basin City. This removed any last rail from Grant County.