New York Central's Putnam Division
The Putnam Division was first organized in 1869 as the New York and Boston Railroad. After a series of name changes, consolidations and bankruptcies, it opened in 1881 as the New York City and Northern Railroad. A complete timeline of events affecting the Putnam Division is here.
Remaining Structures
Here are some pictures of some of the structures I've found along the former route of the Putnam Division railroad grade:

Elmsford station — This station is on NY Route 9A just south of NY Route 119, and is currently being used as the Casaletto Restaurant.

Railroad bridge at Eastview -- Before 1931, the Putnam Division used to loop around what is now the Tarrytown Lakes. The entire town of Eastview was obliterated when John D. Rockefeller, Jr. had the Putnam Division relocated from his Pocantico Hills estate. This is a bridge that was built for the 1931 route.

Old boxcar at Eastview — This old rusting boxcar sat forgotten on a siding at Eastview and was eventually removed in the 1980s.

Briarcliff Manor station — Walter W. Law (who built Briarcliff Lodge) donated this Tudor-style station to Briarcliff Manor in 1909. It is currently used as a library.

Millwood station — When Briarcliff Manor station was built, the old station was moved a few miles up the line to Millwood. Sadly, this station was demolished in May 2012.

Railroad bridge over Croton Reservoir — Much of the Putnam Division has been paved over for use as the North County Trailway bike path.

Yorktown Heights station — A turntable and freight yard were located just south of the station.

Caboose at Yorktown Heights — This Penn Central caboose (PC 19123) was built in 1916 and was originally in service around Pittsburgh. It was donated to Yorktown Heights in 1977, and then moved to the Danbury Railway Museum for restoration in 1997.

Baldwin Place freight house — The station is gone, but the freight house from this station still exists.

Lincolndale station — Lincolndale Station was built in 1915 and was actually located on the Harlem Division's Mahopac Branch line, which connected with the Putnam Division near Baldwin Place. The Mahopac Branch was 7.22 miles long and carried freight and passengers from Golden's Bridge to Lake Mahopac.
Additional Information
Other sites with additional information about the Putnam Division:
- Web-Pages dedicated to the Putnam Division - A listing from 2006.
- "It is contemplated to amalgamate" - A chapter covering the early history of the Putnam Division.
- New York Central: Putnam Division - Pierce Haviland's page.
- Putnam Division as Commuter Rail? Viable? Needed? - A discussion about reopening the Putnam Division.