Release the train from the lift hill. Raise or lower H, change the loop radius and friction, and watch speed, g-load and energy.
The science
Energy. Without losses the speed at height y is v = sqrt(v0^2 + 2 g (H - y)), independent of the track shape. Rolling friction (mu times the normal force) and air drag (1/2 rho CdA v^2, CdA = 3 m2 here) turn mechanical energy into heat; the energy chart shows where it goes.
Rider load. Along a track of curvature kappa and tangent angle theta the normal reaction per unit weight is N/mg = (v^2 kappa + g cos theta) / g. At the bottom of a loop of radius R it is 1 + v^2/(gR); at the top it is v^2/(gR) - 1, which drops to zero (the cart leaves the track) when v^2 = g R. Energy conservation then gives the minimum release height H = 2.5 R for a loop from rest.
Numerics. The track is sampled every 5 cm; tangent and curvature come from the polyline. The cart is integrated along arc length with a 2 ms velocity-Verlet step. Four independent checks (bottom speed, loop-top load, minimum height, energy drift) run in the checks panel.
Simplifications. A single point mass, constant friction coefficient, no wheel rotation or track flex, no lateral forces; the loop is a perfect circle, so in the side view its entry and exit overlap (a real layout offsets them sideways). Real coasters use clothoid loops to limit peak g.