The success of creosoting depends almost entirely upon the effectual seasoning of the timber. Only a very small quantity of creosote can be forced into wet or unseasoned sleepers, even with the best machinery and exceptionally high pressures, while
a thoroughly dry sleeper will readily absorb from 2⅓ to 3 gallons. More could be forced into the dry sleeper if necessary, but a little consideration will show there would be no advantage in doing so. In railway sleepers there are two elements of destruction at work—one the decay of the timber, and the other abrasion or wearing away of the wood itself from the constant pounding of the passing loads.
More particularly does this wearing-away take place with the flange, or bridge, rails, their distributed bearing surface on the sleeper being less than the cast-iron chairs.
A thoroughly well-creosoted 5-inch sleeper laid originally with a thickness of 4-¾ inches in the centre of rail-seat, as in [Fig. 306], will wear down 1½ inches, the timber remaining quite sound.
The writer has had to take out thousands of sleepers where the seats of the flange, or bridge, rails had been pounded or worn down so deep into the wood as to leave too small a thickness of timber to carry the rail with safety. These sleepers had to be taken out of the road, not on account of decay, but because they were actually worn down too thin to be of service. They had done their work well for a long series of years, and were perfectly sound when taken out. No increased quantity of creosote would have made them last longer, and any increased quantity of creosote would have been waste.
Two and three quarter gallons of creosote is a very good and suitable quantity for a 10 inch by 5 inch rectangular sleeper, but not more than half this quantity can be forced in if the sleeper is wet or unseasoned.
Sleeper-blocks are generally cut from the upper part of the tree, and do not therefore consist of the best portion of the timber, yet sleepers made from the soft, coarse-grained Baltic wood, properly creosoted, will last from twelve to eighteen years in the line in this country, while uncreosoted they would perish from decay in six or seven. The benefit is great when, by adding from eightpence to a shilling for the cost of creosoting, the life of the sleeper may be doubled or trebled. Of course, there are countries, like the far west of America, where the lines pass through vast forests, and where sleepers may be had for the mere cost of cutting. Creosoting in those places would be out of the question, and would cost four or five times the value of the plain sleeper. It is found, also, that in tropical countries and
in dry climates at high altitudes creosote loses its efficiency, and in those districts the best creosoted soft-wood sleeper perishes from a species of dry rot in three or four years. Where wood sleepers have to be used in tropical climates it is better to obtain them from the timber of the district, although in many cases suitable trees are difficult to procure and the cost of land transport is very heavy.
The soft cushion-like effect of a sound, properly packed wooden sleeper contributes so largely to form an easy, smooth-running road, that so long as they can be obtained at a moderate cost, and are fairly durable, wooden sleepers will always be preferred to those of any other material. The great question will be the supply. Creosoting and other wood-preserving processes have done much to prolong the life of sleepers, but the rapidly increasing extent of mileage throughout the world, together with the enormous number of sleepers required annually for maintenance or renewals, must before very long severely tax the powers of supply.
In the great timber-producing territories the axe is often heard, but the planter is rarely seen. Vast forests are cleared away, and their sites transformed into busy towns or cultivated lands; and unless some great change takes place, and planting be carried out on a large scale, some other material will have to be adopted for this important item of our permanent way.