after by collectors, as are the extremely rare mechanical dwarf semaphores of the T. George Stiles Company. These signals were installed at the beginning of the 20th century by the New Haven Railroad and used into the 1980s. Common signal rule classes Most North American railroads have between 10 and 20 separate signal rules, each which are often represented by multiple aspects. However, all of these complicated rules revolve around the simple premise of informing the locomotive engineers how they are to operate their train in the present location, and what they are to expect at the next signal location. From here the large set of rules and aspects can be broken down into a small number of classes which are common to all North American signaling systems: Automatic Block - Block aspects convey basic track occupancy information and advise the engineer (operator) which of the basic signal rules (common to all railroads) he/she is to follow in the operation of his/her train at any point on the railway line. These include Clear (ARA Rule 281, see Figure), Advance Approach and Approach (Rule 285) which instruct the engineer to "expect no stop", "expect stop at second signal" and "expect stop at next signal" respectively. Advance approach is only used in situation with short signal blocks to ensure trains have enough stopping distance. These are the most common signal aspects in North America and are the only aspects most automatic block signals need to display. Approach at Speed - When a train needs to be told to slow down due to dynamic conditions an "Approach Speed" aspect is used. These inform the engineer to slow to a prescribed speed by the next signal. The most common reason for this is that the train is to take a diverging, or non-Normal speed route at the next interlocking. Signals of this type include Approach Medium (ARA Rule 282, see Figure), Approach Limited, Approach Slow (Rule 285) and Approach Diverging. These signals are typically displayed on the distant signal to an interlocking, but can sometimes be used with short signal blocks in place of Advance Approach. Diverge to Clear - This class appears only on absolute signals and informs the engineer that the train will be taking a diverging route and need not expect a stop at the next signal. In speed signaling the engineer is informed of the speed the train needs to