Bhramarraj, look at the 4-car case 2:3:4:6.
Clearly the 2, 4, and 6 represent the 1:2:3, and we've inserted one between the first two.
When looking at a pair of cars, I like to divide their speeds by the smaller.
So, looking at the 3:4 case, I think of that as 1:1.33. This means, when the slower one has reached the intersection for the first time (1), the faster has gone one third of the way around the loop beyond. When the slower one has reached for the second time, the faster is now 2.66, and when the slower reaches for the third time, the faster is reaching it for the fourth time.
The other cases are familiar from the 1:2:3 situation.
As it turns out, to get more than 4 cars with compatible speeds, we have to abandon the 1:3 ratio.
Is this a useful explanation?