Railroad Ballast: Rock Sizes, AREMA Grades and Tons Needed

Railroad ballast is the layer of sharp, fist-sized rock you see packed around railroad ties. It looks like a simple pile of stone, but it is a carefully specified product. Ballast carries the weight of trains down into the subgrade, keeps the ties from sliding, and drains water out of the track structure fast. Because of those jobs, railroads buy hard, clean, angular rock in tight size bands. The same rock is also sold to the public as ballast stone for drainage, erosion control and rough base. This guide covers what ballast does, the common AREMA sizes, the rock types used, other uses and how to estimate an order.

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Quick facts

What it is: clean, angular crushed rock with very few fines.
Main jobs: spread load, hold ties in line, drain water.
Common sizes: AREMA No. 4A (about 2 to 3/4 inch), No. 4 (about 1 1/2 to 3/4 inch), No. 5 (about 1 to 3/8 inch).
Typical rocks: granite and trap rock, plus hard limestone, quartzite and slag where approved.
Under it: a finer, well-graded subballast layer that separates ballast from the soil.

What ballast does in a track

A rail track is a layered system. The rails sit on ties, the ties sit in ballast, the ballast sits on subballast, and that sits on the subgrade soil. The ballast has to stay open so water drops straight through. If fine material builds up between the stones from crushed rock, windblown dirt or mud pumped up from below, the ballast is said to be fouled. Fouled ballast holds water, loses strength and lets the track settle out of line, so railroads clean or replace it on a cycle. Research for Transport Canada published by TRB tied ballast selection to rock type, wear resistance and expected annual tonnage.

AREMA ballast sizes

Gradation	Nominal size	Typical use
No. 4A	2 in down to 3/4 in	Mainline track
No. 4	1 1/2 in down to 3/4 in	Mainline and branch lines
No. 5	1 in down to 3/8 in	Often yards and walkways, where smaller stone is easier underfoot

Each gradation sets the percent passing a series of square-opening sieves, so a quarry has to screen tightly to make it. For how those sieve limits are checked, see our sieve analysis guide. The closest everyday product is #4 stone, which is similar in size but usually made to a construction aggregate spec rather than a railroad one.

Why the rock type matters

Ballast is hammered by every passing axle and by tamping machines that pack it under the ties. Soft rock breaks down into fines and fouls the track. That is why ballast specs include toughness and durability tests such as the Los Angeles abrasion test and soundness checks, plus limits on flat and elongated pieces. Hard igneous rock such as granite and trap rock is the usual choice on busy lines.

Other uses for ballast stone

Drainage: French drains and soakaways where you want big, open voids.
Erosion control: lining small ditches and culvert outlets. For heavier flows, step up to riprap.
Construction entrances: a stabilized pad of large angular stone that knocks mud off tires.
Driveway bottom layer: over soft soil, capped with finer stone. On very soft ground, consider breaker run or a geotextile underneath.
Garden railways and landscape edging: smaller sizes look the part next to old ties.

How much ballast do you need?

Find the volume, then multiply by the weight of the actual product. A made-up example:

Area: 12 feet wide by 50 feet long, 6 inches deep.
Volume: 12 × 50 × 0.5 = 300 cubic feet, or about 11.1 cubic yards.
If the quarry quotes 1.4 tons per cubic yard (an assumed figure for this example), that is about 15.6 tons.
Add a little for loss into soft ground and uneven grade, and round up to a full truckload.

The crushed stone calculator and gravel calculator run the same math with your sizes, and the trap rock driveway calculator suits hard basalt-type stone. Our guide to unit weight of aggregate explains why tons per yard varies.

Buying tips

Ask for the gradation and rock type, not just "ballast."
Used ballast pulled from old track is sometimes sold cheaply; it may be fouled with fines and could carry oil or chemical residue, so ask where it came from.
Wear gloves; fresh angular rock has sharp edges.

Questions people ask

What is railroad ballast?

It is the bed of crushed rock under and around railroad ties. It spreads train loads into the ground below, locks the ties in line and lets rainwater drain away quickly so the track does not sit on wet, soft soil.

What size is railroad ballast?

Mainline ballast is coarse. AREMA's No. 4A gradation runs nominally from 2 inches down to 3/4 inch, No. 4 from 1 1/2 inches to 3/4 inch, and No. 5, a smaller size often used in yards, from 1 inch to 3/8 inch.

What rock is railroad ballast made from?

Hard, durable rock that resists breaking and wear: commonly granite, trap rock and other igneous rocks, plus hard limestone, quartzite and steel slag in some regions. Railroads test it for abrasion, soundness and particle shape before accepting a source.

Why is ballast angular instead of rounded?

Angular faces lock together under load and hold the ties in place. Rounded river gravel would roll and let the track shift sideways.

Can I use railroad ballast for a driveway?

It can work as a coarse, free-draining bottom layer, but it is uncomfortable to walk on and too open to use as the top surface. Cap it with a finer crushed stone that has fines so the driveway packs and rides smoothly.

How much does railroad ballast weigh?

It depends on the rock type and the gradation, so there is no single number. Ask the quarry for the tons-per-cubic-yard figure for the exact product, and use that in your estimate rather than a generic value.

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