In excavating the top center heading there is, of course, another rise to its floor from the floor of the upper part of the section. Where, as is usually the case in soft soils, this top heading is not driven very far in advance, the earth from the front is usually conveyed to the rear in wheelbarrows, and dumped into the cars standing on the tracks below. In firm soils, where the heading is driven too far in advance to make this method of conveyance adequate, tracks are also laid on the floor of the heading, and an inclined plane is built connecting it with the tracks on the next level below. In place of these inclined planes, and also in place of those between the floor of the tunnel and the level above, some form of hoisting device is sometimes employed to lift the cars from one level to the other. There are some advantages to this method in point of economy, but the hoisting-machines are not easily worked in the darkness, and accidents are likely to occur. [Illustration: FIG. 73.--Diagram Showing Sequence of Excavation in Modified Belgian Method.] In the advanced top heading and in the upper part of the section narrow-gauge tracks are necessarily employed, and these may be continued along the floor of the finished section, or the permanent broad-gauge railway tracks may be laid as fast as the full section is completed. In the former case the permanent tracks are not laid until the entire tunnel is practically completed; and in the latter case, unless a third rail is laid, the loads have to be transshipped from the broad- to the narrow-gauge tracks or _vice versa_. It is the more general practice to use a third rail rather than to transship every load.