Two movements, not one
Put a rigid body on two pivots and set it on an arc and two things happen at the same time. The pivots stay on the track, so the straight line between them becomes a chord, and a chord sits inside its arc. The middle of the body therefore moves toward the inside of the curve by the versine of that chord. Meanwhile the ends of the body stick out past the pivots along the same straight line, and a straight line extended past a chord leaves the arc on the outside.
With the defaults — 22 in radius, 8.5 in truck centres, a 10.6 in body 1.45 in wide — the middle comes in 0.41 in and the outer corner goes out 0.21 in. Which of the two is larger is not fixed: they come out equal at a body length of four times the square root of the radius times the mid-body swing, which for this car on this radius is 12.08 in. A shorter body is middle-dominated, a longer one is end-dominated, and at 10.6 in this one sits on the middle-dominated side. The trouble is that the two point in opposite directions and belong to different cars.
The gap closes from both sides
On the straight, two tracks at 2 in centres leave 0.55 in of daylight between two 1.45 in bodies. Set the same two tracks on concentric curves and the car on the inside track pushes its corners out into that gap while the car on the outside track pushes its middle in from the other side. At the default figures the two effects come to 0.59 in between them, which is more than the 0.55 in you started with: the arithmetic puts the two bodies in the same space by about four hundredths of an inch.
That is not a verdict on your layout. Bodies have taper, couplers let cars pull apart, and nobody runs two 10.6 in cars past each other at the exact worst point on purpose. It does explain why the widened centres on curves are a standing feature of every track standard anybody has ever written, and it puts a number on how much widening this particular pair of cars is asking for.
The couplers swing further than the body
The coupler face sits further from the truck centres than the body end does, and swing out grows roughly with the square of that distance. At the defaults, the body end is 0.23 in out on the centreline and the coupler face at 11.5 in over couplers is 0.34 in out, half again as much. That difference is why the coupler is usually what fouls a scenic edge or a tunnel portal first, and why body-mounted couplers on long cars are the ones that need the wide radius.
What the easement moves
An 8 in easement into a 22 in radius shifts the arc 0.12 in toward the centre of the curve, and the tangent carries on about 4 in past where the old curve started. That is the practical reason easements have to be planned rather than added: the curve is no longer where it was on the plan, and anything drawn tight against the inside of it — a platform edge, a retaining wall, the next track in — has moved by the offset.
What this cannot see
Everything here comes from four dimensions of a rigid box. It does not know about truck side play, about a body that flexes, about diaphragms, about a pilot that projects below the body line, or about the way a coupler swings on its own arc independently of both cars. A real car needs at least this much room and usually a little more. Measure the widest thing on the car, not the width printed on the box.
Questions people ask
How much does a car overhang on a curve?
Two amounts at once. The middle of the body swings toward the inside of the curve by the versine of the truck chord, and the ends swing outward past it. With a 10.6 in body on 8.5 in truck centres at 22 in radius, that is 0.41 in in at the middle and 0.21 in out at the outer corner. Which is larger depends on how far the body overhangs its trucks: on this radius the two are equal at a 12.08 in body, and anything longer is end-dominated instead. Both grow quickly as the radius tightens or the truck spacing widens.
Why do two cars touch on a curve when they clear on the straight?
Because the swings belong to different cars and add. The car on the inner track pushes its corners outward into the gap while the car on the outer track pushes its middle inward from the other side. At 2 in centres with the default car, the two effects eat 0.59 in of a 0.55 in gap. That is why curves get wider track centres than tangent track.
What track centre spacing does a curve need?
More than the straight, by roughly the sum of the two swings. The page works out what spacing would leave the gap you asked for, using your own tolerance rather than a published figure. It reports a number; deciding whether that number is the right one for your couplers, your detail parts and the way you run is yours.
How much does an easement move the curve?
By the easement length squared, divided by twenty four times the radius. Eight inches of easement into a 22 in radius moves the arc 0.12 in inward, and the straight carries on about half the easement length past where the old curve began. Anything drawn tight against the inside of the curve moves with it.
Does this tell me my minimum radius?
No. It gives the geometry — where the corners and the middle of a body of your dimensions end up on a radius of your choosing, and what is left between two of them. Whether that is enough depends on your couplers, your detail parts, how truly your track is laid and what you are willing to live with, and none of those is something a page can decide for you.