The L in reference to the Chicago Transit Authority (CTA) train system is a nickname that stands for elevated.
The meaning behind the nickname L for the CTA trains
The name reflects the system’s early days when a large portion of the tracks were built and operated as elevated railways. Although the network now includes subway tunnels and at-grade sections, the nickname has remained.
The use of L as a shortened version of elevated railroad dates back to the very beginning of the system in the 1890s and was quickly adopted by the public and press. Some sources suggest the official spelling of L (instead of el) differentiates it from the generic term for elevated railways used in other cities like New York City.
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