El tren de cercanías iba más lleno de lo que parecía, pero el trayecto fue corto.

Cercanías literally refers to nearby areas or the surrounding area(s). That is why it appears in the plural. In transport vocabulary, de cercanías has become a fixed expression meaning for local/commuter routes.

So:

• tren de cercanías = commuter train
• not usually tren de cercanía

Here iba is the imperfect of ir, and with vehicles it can sound very natural in Spanish.

El tren iba lleno means something like:

• the train was full
• the train was crowded
• literally, the train was going along full

Using ir gives a sense of the state during the journey. It is very common with transport and movement.

You could also say estaba más lleno de lo que parecía, and that would be grammatical, but iba feels a bit more natural for a train in motion.

This is a classic imperfect vs preterite contrast.

• iba describes an ongoing background situation: the train was more crowded than it looked.
• fue presents the journey as a completed whole: the trip was short.

So the sentence is dividing the experience into:

1. a background description: iba más lleno...
2. a finished fact/result: el trayecto fue corto

That is why the tense shift sounds natural.

Literally, lleno means full. But when talking about public transport, English often prefers crowded or packed.

So:

• El tren iba lleno = The train was full
• more natural in many contexts: The train was crowded

In this sentence, más lleno is best understood as more crowded / fuller than it seemed.

This is a very common Spanish comparison pattern:

más + adjective + de lo que + verb

It means:

more + adjective + than + clause

So:

• más lleno de lo que parecía = more crowded/full than it seemed
• literally, something like more full than what it seemed

You see the same structure in sentences like:

• Era más difícil de lo que esperaba. = It was more difficult than I expected.
• Es más caro de lo que parece. = It’s more expensive than it looks.

Because after a comparative like más, Spanish often uses de lo que when what follows is a whole clause rather than just a word or phrase.

So:

• más lleno de lo que parecía
• más interesante de lo que pensaba
• menos complicado de lo que parece

This is just a standard Spanish structure. English uses than, but Spanish often uses de lo que in these cases.

Spanish often omits subject pronouns when they are understood from the verb form or the context.

In parecía, the subject is an implied he/she/it. Here, English needs it:

• de lo que parecía = than it seemed / than it looked

The implied subject is basically the train or the situation as it appeared. Spanish does not need to say ello or eso here.

Because lleno agrees with el tren, which is masculine singular.

• el tren → lleno
• la estación → llena

So if the sentence were about a station instead, you would say:

• La estación iba/estaba más llena de lo que parecía.

Both can relate to a journey, but they are not exactly the same.

• trayecto = the route or stretch from one point to another; often more concrete and practical
• viaje = a trip more generally

In this sentence, el trayecto fits well because the speaker is talking about that particular ride from A to B. For a commuter train, trayecto sounds very natural.

It can sometimes suggest either, depending on context, but here it most naturally refers to time/duration:

• the train was more crowded than expected,
• but the ride didn’t last long

So el trayecto fue corto is basically saying that the short duration made the crowding easier to tolerate.

Yes. That is a very natural English rendering.

A more literal version would be:

• The commuter train was fuller than it seemed, but the journey was short.

But in normal English, crowded and ride often sound more natural than full and journey in this context.