On 2 January 1902, the railroad was also approved by the
Secretary of the Interior to conduct surveying for a southern line though the Colville Reservation along the
Sanpoil River. From the southeastern boundary of the reservation the railway located a line along the
Columbia River and then up the
Spokane River to a junction with the then defunct
Seattle, Lake Shore and Eastern Railway road bed. Another juncture, with the
Okanogan Electric Railway, was planned near
Peach, after which the two would share the line into Spokane. The mapped route for the southern extension was approved 17 October 1905 was approved by the Secretary of the Interior. Included in mapped route was location of the proposed branch line starting near the mouth of the Spokane River at the Columbia River and thence traveling along the eastern shore of the Columbia to "Sand Bar Landing" which was also the terminus of the
Big Bend Transit Company rail line. The Secretary of the Interior gave notice upon map approval that consideration for the branch line would not take place until construction of the railways main line south had been completed. During the same year, the small
Belcher Mountain Railway had also surveyed a southern line from Republic across the Colville Reservation to the Columbia. The company had been directed that in any case where the proposed route paralleled another rail line within 10 miles, it was to give justification for the benefit of public interest for two lines. Upon review of the proposed route, submitted 24 November 1905, the Secretary of the Interior noted it to cross, re-cross, and at times use the same exact bed as that which had already been mapped by the Spokane and British Columbia Railway. The Belch
Mountain Railway maps were not approved due to failing to meet submission requirements and notice of the conflicting route was given. The full extent of the line was a proposed route connecting Republic to
Spokane, Washington. With the 1905 reorganization of the company into the Spokane and British Columbia Railway, an additional $4,000,000 in stocks was authorized for the southern line. However, as of a 1915 valuation by the US Commerce Commission, little actual building or prep work had been made on the southern route due to building debts, and none of the additional stocks had been issued. On June 30, 1915, the S&BC had a total operating loss of at the end of the 1915 fiscal year. ==Washington & Great Northern Railway rivalry==