The Grimes Branch
This line was originally built by the St. Louis, Des Moines and Northern Railway in 1882. Ownership of the line passed through successive hands via various mergers and bankruptcies/consolidations:
The line was actually abandoned by the CMStP&P in 1980 as part of its bankruptcy reorganization, but was soon brought back to service by the Norfolk and Western in 1988.
The Norfolk Southern abandoned a portion of the right-of-way around Grimes in 2012.
This line went to Madrid Ia first going though Zot Spur. This line crossed the Des Moines river on a high wood bridge . Area around the Bridge was known a High Bridge which was a resort type of town about 1900.
Some time after the low bridge crossing on the Milwaukee was replaced the line was change to the route shown on the map. The first route is about 1/2 mile west of the present highway continued on north to Boone Iowa.
I think you meant Zook Spur, which is just north of the Des Moines River, just west of IA-17. Seems like I saw a panel on the High Trestel Bridge that describes how the rail came down from Madrid (or east of Madrid) to Zook Spur and crossed the river there until they built the higher bridge west of Madrid.
A couple of points about this line. My office is about two blocks from the most northern part of the line that is still active. The line ends at SE 37th Street - with a few hundred yards north of the crossing used for occasional car storage. The Iowa Interstate Railroad actually serves the line and has for years. They installed a run-around track just south of the SE 37th Street crossing in order to better serve the lumberyard at the end of the line.
One more thing - I believe the stone bridge that carried this line over the east-west line just south of downtown Madrid is still standing. You should be able to see it by looking west from Iowa Route 17.