The building of the new station has offered tremendous problems to the engineers, for it has demanded a complete reconstruction within extremely limited area, while not placing hindrances in the way of the constant operation of one of the world's greatest terminals. Coincident with the rebuilding of the new station has come the substitution of electricity for steam on the terminal lines of its two tenants, the New York, New Haven, & Hartford, and the New York Central & Hudson River Railroads. In order to work the three-mile tunnel through Park Avenue and the sole entrance for trains to the station at greatest capacity, it was found necessary to extend the yards of the new station far north of those of the old. This work, alone, has necessitated the acquisition of whole city blocks of tremendously valuable real estate and the excavation of several million cubic yards of rock and earth. To accomplish the work of reconstruction and still enable the station to handle its great traffic without serious interruption, serious forethought and definite plans of action were found necessary. The plan was developed by constructing a temporary building of brick and plaster covering a vacant city block in Madison Avenue, at the west of the station. Into this temporary structure a branch post office, an important adjunct of the Grand Central, was moved from the extreme eastern side of the terminal. Excavation for the new terminal began at its eastern edge and at that edge the first portions of the new structure have been completed. A waiting-room was then established in temporary quarters, the last vestiges of the old Grand Central removed, and the main front and centre of the new station fabricated. Similarly, as the excavation has progressed from the east to the west side of the terminal, the great bulk of the traffic has been gradually shifted from the old high-level to the new low-level. The new Grand Central complete will have its main train-shed devoted to through traffic. A second train-shed of similar arrangement and of slightly smaller dimensions will be constructed underneath the main shed for suburban traffic, and a single head-house will serve both floors. The head-house will have as its chief architectural feature, a concourse of mammoth proportions. The lesser features of the new Grand Central will contribute to make the new terminal, built upon the site of the historic old, one of the world's greatest gateways. The fact that steam locomotives are absolutely prohibited from entering either of the two new stations on Manhattan Island makes these the cleanest railroad terminals yet built.