How does one keep trains from slowing down while waiting for a green signal?

From point A, through B, to C, there is a railway serving a large number of trains.

At point B, a train wants to enter the railway, but stops at a signal as the tracks are currently occupied. Now when it enters, it will do so at a low speed, blocking the entrance for an added period of time, which then blocks the next train, and so on.

What techniques can I use to make point B more efficient, and not slow down a lot of trains?

Cyclotrons are what you want.

http://wiki.openttd.org/Railway_Designs#Cyclotrons

A cyclotron is a loop with a priority merge. A train willing to enter a hi-speed main line enters the loop (preferredly at full speed), and subsequently passes through the priority merges every so often, depending on the size of the loop and train speed. The priority merge lets the train enter the main line if no other train is passing at the time, and continues in the loop otherwise.

This effectively keeps the train at full speed and allows it to enter the main line only if there is enough space between trains there, and at full speed. As a result, the main line never slows down.