Tunnels
Long wet tunnels present peculiar difficulties to the reliable operation of the rail circuit; and yet these difficulties are readily overcome by including one or more additional relays in the signal section, as shown in Fig. 8, which illustrates the application of the Robinson track circuit system as applied to the Tehauntepec tunnel in California.
Mr. Robinson forwarded the signals and necessary instructions, and the installation was made by Mr. Stephen D. Field, secretary of the Electrical Construction and Maintenance Co. of San Francisco.
Fig. 8.
Figure 8 is from a sketch made by Mr. Field in a letter dated San Francisco, March 21, 1877, addressed to Mr. Robinson.
In this letter Mr. Field says: "I am just in the receipt of yours of the 12th. I had anticipated your diagram and have the signals arranged as you show.
"I use the system connected up as follows:
"In the tunnel the rails are buried in wet mud; outside no moisture touches them for six months of the year."
It will be noted that in the above case the signal section is two miles long, the tunnel being one mile long, with its rails "buried in wet mud," and the section extending one-half mile at either end of the tunnel. An extra relay and battery are placed in the center of the section connected up as shown. Thus, where conditions require, a signal section may be divided up into a number of sub-sections.