The engineer has 'bunched' the slack in the train, so that each coupling has the maximum slack distance to move before starting to pull on the car behind it. In the procedure being cautioned, the air brakes are applied on the entire train.
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All engine driven trains are likely to start with jerks because many coaches are coupled and these couplings are not rigid as explained in detail in other answers by experts .
This is due to inertia. Inertia is a property of matter by which it continues in its existing state of rest or of uniform motion unless acted upon by a net unbalanced external force.
As far as I'm aware, there's no legal limit. Passenger trains do not normally exceed 12 cars (around 900 feet, dependent on rolling stock type), but many are much shorter than this.
Railroad tracks guide the train, acting as the low-friction surface on which the train runs and often transferring the weight of the train to the ground below. The track may also provide electrical power along the third rail, as you'll recall.
Wood has the natural flexibility that is perfectly suited for the loads running on railroad tracks every day. Wood ties are engineered to perform under heavy load conditions. The durability of the wood tie means lower costs for railroads.
Rail corrugation (a periodic wear pattern resembling corrugated metal) causes tonal noise and vibration; fine, short-wavelength corrugation is known as roaring rails due to its high-pitched sound, whereas coarse, long-wavelength corrugation can cause the ground and nearby buildings to vibrate.
For a variety of reasons the practice is less common in the 21st century, although a community of freight-train riders still exists. Typically, hoppers will go to a rail yard where trains stop to pick up and unload freight and switch out crew.