Railway-bridge decks may be of timber ties, either plain or treated, spaced from twelve to fifteen inches from center to center, with either two or four rows of wooden guard-rails bolted thereto; trough floors with or without ballast; ballasted road-bed resting on a solid floor of treated planking, steel plate, or reinforced-concrete slab; or rails resting directly on the steelwork.

The most common type, and the cheapest, as far as first cost is concerned, is the open, untreated-timber deck; but it may be more expensive than some of the other types when the item of maintenance is considered. It affords easy riding, but is not so safe against derailment or burning as decks that are closed. These advantages and disadvantages, however, cannot be evaluated in money. Treated ties, of course, cost more than the plain ones; but they generally last so much longer that they are more economical in the end. They can be of a cheaper grade of timber than the untreated ones, but this saving is offset by the necessity for using tie plates between the timber and the rails. Without these the life of the soft timber would be very short under heavy traffic. However, first-class construction calls for tie plates on all timber ties.

Trough floors with ballast and ties therein are uncommon. This was one of the first kinds of shallow floor to be built, but it has gone out of fashion. There was one tie per trough, and it generally rested directly on the steel; but sometimes a few inches of ballast were interposed. This type is noisy and expensive, and the replacement of ties in the trough is both difficult and costly.

Ballasted decks are more expensive in respect to first cost than the open ones, especially in long-span structures, on account of the augmented dead load. They provide easier riding; and for short-span bridges on first-class lines they are almost exclusively adopted as standard. One incidental advantage that they possess is that they permit the use of skew abutments, which are not compatible with the open-timber type of deck. Again, they are more conducive to maintenance of alignment; and they protect the steelwork fairly well from brine drippings.

182