Trail is a little harder to visualize. If you continue the centerline that is the steering axis down until it intersects the ground that intersection describes one end of your trail measurement. Dropping a vertical line through the front axle center to the ground will generate the other end. The distance between these two points is the trail. If the axle center trails the steering axis the trail is positive if the axle center leads the trail is negative.
Positive trail helps make the steering self-center.
A motorcycle has positive rake, as well, which up to approximately 45 degrees uses the weight of the bike to orient the axle center in a trailing position to the steering axis. This eliminates the need to push the motorcycle forward to effect that orientation, as in the caster wheel. Most vintage bikes have trail in the range of three to four and one half inches. Most street bikes are set up with fairly slow heavy steering and a strong rearward weight bias. This method has the added benefit of shifting weight to the front wheel.