Question: What is the literal meaning of "auf der Strecke bleiben"? What is the link between the literal meaning and the meaning "to fall by the wayside"?

Answer: 
Imagine an army making a forced march, maybe being pursued by enemy troops. Some soldiers will run out of energy to march on and collapse so they stay on the track instead of reaching the destination.
As idmean has answered, it literally means

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> to stay on the track
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However, it is not so much about failing but rather about not being completed, being ignored/forgotten or being put at disadvantage, see here: [German examples](https://www.redensarten-index.de/suche.php?suchbegriff=%7E%7Eauf%20der%20Strecke%20bleiben&bool=relevanz&suchspalte%5B%5D=rart_ou)

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> "Mutti bleibt auf der Strecke. Frauen im Erziehungsurlaub drohen beruflich abgehängt zu werden"; \*Mum falls by the wayside. Women on maternity leave are threatened to fall behind in their jobs.
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> "Ihr Geld bleibt auf der Strecke - Wertverlustprognosen für die 100 meistverkauften Modelle"; *Your money falls by the wayside. Lost in value predicted for top 100 sold models.*
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> "Abrüstung bleibt auf der Strecke. An der Jahrtausendschwelle droht neue Runde im globalen Wettrüsten"; \*Denuclearisation stays on the wayside. Threats of new arms race in new millenial.
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> "Deutsche Bahn will sparen: Behinderte Menschen bleiben auf der Strecke"; *Deutsche Bahn wants to save money: Handicapped people stay on the wayside.*
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> "Kino digital: Die Kleinen bleiben auf der Strecke" *Digital cinema: Children stay on the wayside.*
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I delibaretely chose the *wayside* translation, although you probably wouldn't say it this way in English. If needed, I can provide proper translations.