On the South Devon Railway we have two sections; Buckfastleigh to Staverton (Bishop's Bridge) and Staverton (Bishop's Bridge) to Totnes Littlehempston (Ashburton Junction).
The first section has two operational 'boxes and so can have trains moving in the same direction, one after the other, once the first has safely arrived at Bishop's Bridge.  The lower section to Totnes and the National Rail network doesn't yet have an operational 'box at Totnes Littlehempston and so is "one engine in steam" working.

Ever wondered how a signalman remembers which levers to pull to put a train into the correct place and why they don't go wrong (normally) ?
Well it's down to good training and a basic function called interlocking where the selection of one signal or point movement prevents an opposing move from being selected.

Above is a simple track layout with the points and signals marked on it and a table of what the levers do together with a list of which levers have to be moved before the chosen lever can be moved.
The role of a Signalbox is very important since it can affect the safety of the public and the trains.  For this reason all efforts are put into training the Signalman to do the job correctly the first time but just in case he gets distracted then the signal frame will have been set up to prevent dangerous options being selected.
Over the main signal frame is a map, similar to the one above, of the section of track which the signalbox controls and any instruments that help the Signalman with his job.  The most basic are the bells for the signalboxes on each side of him.  Others are repeaters for signals he cannot see so he knows that they're displaying the correct aspect (red or green for a stop signal (yellow or green for a distant)).
There are a number of locking styles used to restrict the movement of signal and point levers but the Great Western Railway tended towards the tappet type as illustrated below.

In the example above the signalman cannot allow a train through the station until the road crossing gates have been closed and locked by moving lever 1.  The pedestrian wicket gates are left until nearer the time and are locked when necessary by 2 and 3. With the road crossing made safe then the stop signals 5 & 6 and or 24 can be set to "Off"nbsp; Obviously to release the "UP Main Outer Home" signal (5) the "West Crossover" controlled by lever 9 has to be in the straight through position so lever 9 can stop lever 5 being moved if this is not the case.  The same is true for the "East Crossover" lever 17.
If you study the diagram above and the "locked by" table you can see how the setting of points to achieve a move is either prevented by the frame locking from achieving a potential conflict or, if their movement is allowed, they then lock other levers to stop new potential hazards.
The above is based on a 1956 decription of signal locking from a series of lectures for the Institution of Railway Signalling Engineers and wouldn't comply with modern requirements; particularly the pedestrian wicket gates.

A signalman never acts alone in the sense that he always has to ask the next signalbox along the line if he could send a train into the next section before he allows it out of his section of control.  Obviously he needs to accept trains from the preceding section if  it is safe to do so.
To aid rapid confirmation of the expected action all levers in a signalbox are painted in a distinctive colour which reflects their purpose as detailed in the table below: