The inherent problem for bicycles of tipping over the handlebars when the front brake is applied suddenly is less of a problem on a tricycle, as the weight of the axle tends to counterbalance the rider. The worst normally experienced is a jolt as the rear returns to earth. Due to the extra wheel, there is little danger of a front wheel skid causing a fall.
Front Wheel Braking
The majority of tricycles tend to have two brakes on the front wheel, the set-up can take many forms.
The simplest, especially if adding a tricycle conversion set to a bicycle frame, is to use a hub brake for the second brake. Older hub brakes have not generally been very efficient, useful mainly to gently slow down the machine, leaving any emergency stopping to the rim brake fitted as for a standard bicycle. Modern hub brakes seem to be much more powerful.
Traditionally tricycle frames have been built with a ‘nose’ protruding from the fork crown enabling a brake to be mounted at the end, in front of the second, cantilever brake brazed onto the forks.
With modern ‘mini-V’ brakes available nowadays, there is a growing trend to mount one in front and one behind the front forks.
Braking on the Rear Wheel(s)
Nothing is impossible, and there are ways of providing braking to the rear of the machine.
Tricycles can be ridden on a fixed gear which, on a single-wheel drive provides braking on the left-hand wheel. When racing this can often require some extreme acrobatics to keep the balance under control. Before the advent of modern two wheel drive systems, a similar effect was achieved using a differential as used in motor vehicles. A differential enables the rider to use a fixed gear to brake on both rear wheels.
Hub brakes can also be fitted to the rear wheels, and often are added to complement two front brakes on a tandem-trike.
Mountain biking has brought disc-brakes to the world of cycling, and these are often fitted to trikes, both at the front and/ or rear. At the rear they seem to be normally mounted just in-board of the wheels, the mechanism being anchored to the axle. There are tricycles with discs mounted behind the gear block, though I believe there have been reports of broken axles due to excess torsion, and the proximity of the braking surface to all that grease sound dubious.
Finally, it is possible to mount caliper brakes on a bar over the rear wheels. This is a simple enough solution, but carries the disadvantage of increasing the weight of the machine.