The most common and most likely answer to this crossword clue is the 8 letter word SWITCHES
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More clues leading to the result SWITCHES
Buddy explains!

The straight part
We know that "rail-yard controls" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rail-yard controls = switches.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SWITCHES mean?
SWITCHES noun- A bundle of thin sticks, typically made of wood, sometimes bond in such a way that binding can be moved so that it varies the tightness of the binding.
- A device to turn electric current on and off or direct its flow.
- A change or exchange.
- A movable section of railroad track which allows the train to be directed down one of two destination tracks; (set of) points.
- A slender woody plant stem used as a whip; a thin, flexible rod, associated with corporal punishment in the United States.
- A command line notation allowing specification of optional behavior.
- A programming construct that takes different actions depending on the value of an expression.
- A networking device connecting multiple wires, allowing them to communicate simultaneously, when possible. Compare to the less efficient hub device that solely duplicates network packets to each wire.
- A system of specialized relays, computer hardware, or other equipment which allows the interconnection of a calling party's telephone line with any called party's line.
- A mechanism within DNA that activates or deactivates a gene.
- (BDSM) One who is willing to take either a submissive or a dominant role in a sexual relationship.
- A separate mass or tress of hair, or of some substance (such as jute) made to resemble hair, formerly worn on the head by women.
- To exchange.
- To change (something) to the specified state using a switch.
- To whip or hit with a switch.
- To change places, tasks, etc.
- To get angry suddenly; to quickly or unreasonably become enraged.
- To swing or whisk.
- To be swung or whisked.
- To trim.
- To turn from one railway track to another; to transfer by a switch; generally with off, from, etc.
- To shift to another circuit.