If I am guessing correctly, the building being moved above is one at the Sturtevant factory. The horses appear to be harnessed to capstans to apply the pulling force.
Edit: You can see how buildings were moved by horses here.
Boston Daily Globe July 10, 1895
Away With Grade Crossings
Details of the Work Commenced by NY NH & H Railroad Company.
Starting at West Chester pk, the New York, New Haven & Hartford railroad have commenced the abolition of every grade crossing, eight in all, from this point to Forest Hills. The lowering of Center st at Hoggs bridge, below the track of the road and the widening of other thoroughfares, is also planned.
A conservative estimate of the cost of this great undertaking is $3,000,000. Of this amount the railroad pays 65 percent, the state 25 percent and the city 10 percent.
It is going to take about two years to do the work, and during this time the railroad company expects to so arrange matters that their traffic will be uninterrupted.
Since May there has been great activity all along the main line of the road, on the points named, and officials of the road have been busy adjusting claims of abbutters whose property has been taken. The railroad officials say that they have dealt with all property owners liberally, in most cases paying two and one half times the assessed value.
More than 100 houses and business buildings have been taken for the purpose of widening the roadbed, and along Boylston av, in Jamaica Plain, it has been found necessary to acquire the entire westerly side of the street.
The railroad is doing it own filling and grading, as this has got to be done between trains and it is not considered advisable to allow contractors to have a hand is such a delicate matter. The rest of the work will be done by contract, however, and already two contracts aggregating $1,250,000 have been awarded New York firms.
Engineer Ingersoll, who has charge of the job, has established headquarters in Jamaica Plain, where he has located his efficient force of engineers.
Starting at West Chester pk, there will be an easy rise in the grade of the road. As the tracks approach the present roundhouse, it will be about five feet higher than now, but there will be no difficulty running into the roundhouse on this account. Workmen by the score are now getting ready for the elevation at this point.
The roundhouse on the baseball ground side of the road will be discontinued.
Continuing along to Ruggles st, the tracks of the roadway will pass over the street by means of a steel girder-plate bridge, which will have 13 feet head room. At Prentiss st the tracks will pass over a bridge of similar design, (?) feet wide. All along here small strips of property have been taken to widen the roadbed.
At Roxbury there will be a wonderful geographical and architectural change. Here will be an inward and outward station, and Tremont st at this point will be about 100 feet wide. Forty-foot driveways will approach the station on either side independent of Tremont st, and one may cross from one station to the other by an underground passageway. When these improvements are carried out trains will run to the right, instead of to the left, as is now the case.
Between Roxbury crossing and Heath st a lot of property has been taken in order to carry out the improvements.
Old Heath st will be crossed by a 50-foot steel bridge, and the present dingy station will give way to two new brick structures.
Just above Heath st is Hoggs bridge. Center st now runs over the railroad, but under the new plan the railroad will run over the new Center st, which is to be widened to 80 feet. This is considered a great improvement to property in this section.
The old freight yard at Heath station is to be transferred to Lamartine st, just beyond Hoggs bridge, and work of filing in at this point is now going on. Land on both sides of the railroad has been taken at these points.
At Boylston station there will be a great change. Buildings have been removed and another station will be built, and a bridge across boylston st is planned.
Old landmarks like D.A. Brown's store at Green st, Jamaica Plain, have been torn down, and the great wooden building, Bartlett's block, across the street, is to be removed. Two elevated stations are to be built at Jamaica Plain also.
Between Boylston station and Forest Hills there is more building moving going on just now that in any part of Boston. It is a veritable harvest for the building movers, who can command their own price for the work. The biggest job that is going on in this line is the moving of a big brick factoryy at Jamaica Plain. It is 150 feet long and 50 feet wide, and requires 10 horses to furnish the motive power. Generally one or two horses suffice to move ordinary buildings.
Beyond this point Keyes st will be crossed by one of the steel girder bridges, similar in design to the others mentioned. At Morton st there will be a beautiful masonry five-arch bridge under which will pass the parkway, connecting Franklin Park with the Arboretum, and just beyond this will be another steel span bridge over Washington st, Forest Hills.
The branch will be three-tracked to a distance of 2000 feet toward Roslindale with a gradual elevation of the tracks until Forest Hills is reached. Hundreds of workmen are now employed at this point pushing the work along. The old Forest Hills station has been moved back, and two stations, one on either side, will be built.