Sharing locomotives

Did steam engines ever venture off the owner railroads tracks? Today you see foreign diesel power on all of the class 1 railroads, would this have ever occured during the steam era?

Technically it happened all the time for short distances: any "Union Station" had locomotives of several railroads pulling trains into it!

Inter-line use of locomotives in the steam era wasn't as widespread as now for a number of reasons. Very few steam runs were more than a few hundred miles long -- steam locomotives needing servicing more frequently than diesels -- and running the same locomotive for very long, inter-line, runs wasn't practical. Second factor: modern diesels of different railroads are very similar to one another (beneath the paint job), so there is no technical barrier to using one railroad's locomotive on another railroad's track, but steam locomotives were not nearly as standardized.

Dont forget about railroads leasing power from other roads if they have a motive power shortage. PRR leased some Santa Fe steamers in the 50's.

IIRC the Erie and NYS&W shared/leased steam locos, though I think it was when the Erie owned the SusieQ?

Not sure but I think the Rock and SP had some 'shared' locos also?

From the 1930s until the Maine Central ended passenger service it had a power pooling arrangement with the Boston and Maine between Bangor, Me and Boston. While there are plenty of diesel photos, steam photos are not all that common, but the Maine Central's dinky Hudsons did get to Boston. Freight and passenger steam power was pooled by B & M and Canadian Pacific between White River Jct and Newport, VT. The Rutland and B & M pooled steam passenger power between Troy, NY and Rutland, VT.One B & M round-trip passenger train utilized a Central Vermont steamer between Springfield, MA
and White River Jct. This was because all of the passenger service was managed by the B & M but two tickets were lifted for the 123 mile run. 109 miles for B & M and 14 miles for CV. Rather than do day by day calculations for an operation that had large seasonal swings in passenger traffic (lots of colleges on line, heavy summer traffic) B & M and CV worked out a deal that CV would provide an engine, some cars and a crew part of the year for all the CV passengers carried those 14 miles (Windsor-White River Jct) on B & M trains. Except for the Bangor-Boston pooling a single crew worked the entire run. Rutland crews operated into Troy, B & M into Rutland, CP crews into White River, B & M crews into Newport, etc. Bangor-Boston crews changed at Portland.

During WWII DM&IR leased it's big Yellowstones to D&RG during the winter when iron ore operations were shut down for 3 to 5 months a year. 1944-45 UP was so desperate for motive power that were leasing and buying everything that wasn't nailed down that was of use to them. They even bought some of N&W's surplus Y class the were WWI USRA locos.

It was not uncommon for railroads to buy or lease surplus locomotives from other lines. Examples I can think of immediately, mostly after WWII:

Boston & Maine 2-8-4s sold to both Santa Fe and Southern Pacific.
Boston & Maine 4-8-2s sold to Baltimore & Ohio.
Seaboard Air Line 2-6-6-4s sold to Baltimore & Ohio.
Atlantic Coast Line 2-10-2s sold to Chicago & Illinois Midland.
Florida East Coast 4-6-2s sold to several railroads.
Boston & Albany 2-8-4s sold to Tennessee, Alabama & Georgia
Boston & Albany 2-10-2s sold to Canadian National.
Monon 2-8-2s sold to Soo Line.
Norfolk Southern (original) 2-8-4s sold to National Railways of Mexico.
New York, Ontario & Western 4-8-2s sold to Bangor & Aroostook
Wabash 2-10-2s sold to Chicago & Illinois Midland.
Pennsylvania Railroad borrowed Santa Fe 2-10-4s on the Sandusky line in the late steam era.

I am sure many more could be added to that list. I have probably forgotten other examples, and have left off other examples of smaller and older power that I have come across.

Within the New York Central System:
B&A 2-8-4s moved to other NYC lines.
B&A 4-6-2s transferred to Pittsburgh & Lake Erie.
P&LE 2-8-4s moved to other NYC lines (Ohio Division).
NYC L-3 4-8-2s were assigned to the B&A.

Within the Southern Pacific family:
T&NO and Cotton Belt 4-8-4s wound up on the SP proper.

Within the Canadian National system:
Grand Trunk Western U-3-a 4-8-4s wound up on the Canadian National.

Additionally, the Boston & Maine and Canadian Pacific shared power on the line north of St. Johnsbury, Vermont. During WWII a freight train might have a CPR 4-6-0, a CPR 2-8-0, and a B&M 2-8-0 triple-heading (I saw them there as a child).

So, to summarize, despite the non-standardization of steam between most railroads, a good deal of shifting power back and forth took place.