The SP&S really wasn't even a bridge road, but a subsidiary of the GN and NP, built to compete with the UP's access to Portland. Neither road wanted to pay the entire cost, both were owned by Hill and Co, so they jointly financed the line.
Until the BN merger it was owned by the GN and NP. The CB&Q was as well, but it started as an independent railroad, and always had more freedom than the SP&S.