Learn Russian: Каждый день поезд едет через этот тоннель. - Every day a train goes through this tunnel.

The preposition через (through / across) always takes the Accusative case. Because тоннель is a masculine inanimate noun, its Accusative form (and the matching modifier этот) looks exactly identical to the Nominative (dictionary) form.

The sentence says the action happens Каждый день (every day). Why does it use the one-way verb едет instead of the repeated-action verb ездит?

This is a classic exception! Normally, repeated actions require a multidirectional (round-trip) verb like ездит. However, for scheduled public transport (like trains and buses) following a fixed route in a specific direction, Russian often uses the unidirectional verb едет. It focuses on the specific one-way path the train takes through the tunnel on its scheduled route.

Why is there no preposition like в (in/at) before Каждый день?

To express 'every' plus a time period in Russian, you simply put the phrase directly into the Accusative case without using any preposition. Since день is masculine and inanimate, каждый день stays exactly as it looks in the dictionary.

Is the word тоннель masculine or feminine?

It is masculine, despite ending in a soft sign (-ь). You might also occasionally see it spelled as туннель. Both spellings are absolutely correct and mean the same thing, though тоннель is more standard in modern Russian.