that its upkeep and replacement are expensive. If funds for the construction are limited, it may be best to adopt timber trestle-work in spite of its being ultimately uneconomic, with the expectation of saving from the traffic receipts enough money to substitute later on, when replacement becomes necessary, the most desirable type of construction.

Of the permanent types of approach, the embankment is the cheapest where the property-cost is little or nothing, excepting when the grade line is very high or the earth difficult to obtain and, therefore, expensive.

When property is costly or side slopes are not permitted, it is economic for comparatively-low grade-levels to adopt earth embankment between retaining walls.

As the height increases, it becomes cheaper to pass from embankment to trestlework; and the point of division is not difficult to determine when there are no side walls, but when these are requisite, it will be necessary, as previously mentioned, to make the determination by actual cost-estimates. The wider the approach is at grade surface, the greater, for economy, will be the limiting height of embankment.

For low trestles of permanent construction, reinforced-concrete will generally prove economic, but for high ones it will be found necessary to employ steel.