The "Beep" (also referred to as the SWBLW) is an individual switcher locomotive built in 1970 by the Atchison, Topeka and Santa Fe Railway at its Cleburne, Texas, workshops.

Following its successful CF7 capital rebuilding program, the company hoped to determine if remanufacturing its aging, non-EMD end cab switchers by fitting them with new EMD prime movers was an economically viable proposition.

67729 emerged from the Baldwin Locomotive Works Eddystone, Pennsylvania, facility in July 1943 dressed in the Santa Fe Zebra Stripe livery and bearing #2220.

Much as with the CF7 conversions, the unit was stripped down to its bare frame, and the long hood, 1,000 hp (750 kW) power plant, trucks, control gear, and electrical system scrapped; only the Baldwin cab remained.

The completed Beep rolled out of the Cleburne shops in December 1970 (with one of its original Baldwin builder's plates still affixed) sporting fresh blue and yellow paint, though now the words "Santa Fe" were applied in yellow in Cooper Black font (logotype) along the sides below the accent stripe in the style otherwise reserved for road switchers and other main line locomotives.

After Santa Fe merged with the Burlington Northern Railroad in 1995-1996 to form BNSF Railway, the Beep was one of only three blue and yellow units not affected by the new company's renumbering program.

Santa Fe had designated a handful of other non-EMD switcher locomotives for rebuilding around 1970 (including two Fairbanks-Morse H-10-44s), but all of these units were subsequently scrapped when it was determined that the required modifications were not cost effective.