Viajar en tren es seguro, pero hoy el servicio es peor que de costumbre.

In Spanish, when talking about the means of transportation you use en, not por.

• en tren, en coche, en avión
  por would indicate a route (e.g. viajar por Europa) or a reason/motive, not the vehicle itself.

Here, seguro means “safe” or “reliable.”

• es seguro = “it is safe” (adjective after ser)
  You’re making a general statement: “Traveling by train is safe.” In Spanish, state-of-being adjectives (like seguro, fácil, rápido) follow ser.

El servicio here means the quality or reliability of the train system that the company provides:

• punctuality of trains
• frequency of departures
• cleanliness, staff performance, etc.
  We’re not complaining about an individual train, but about the overall “service” that runs the trains.

de costumbre is an idiomatic adverbial phrase meaning “than usual.” You don’t add an article:

• peor que de costumbre = “worse than usual”
  Adding la would turn la costumbre into “the custom,” which isn’t used in this expression.

Yes, both are possible and idiomatic:

• peor de lo habitual
• peor que lo habitual
  But bear in mind:
  • You need the neutral article lo before habitual.
  • de costumbre is more colloquial; lo habitual is slightly more formal.

Spanish adverbs like hoy are flexible. Placing hoy mid-sentence:

• emphasizes the contrast (it’s usually OK, today it’s not)
• avoids repetition at the beginning
  You could also say Hoy el servicio es peor…, but the writer chose this word order for smoother flow.