Learn Hardcore Indonesian: Saya berharap kereta tidak terlambat.

Yes. Saya berharap introduces a content clause, and the clause kereta tidak terlambat uses the normal Subject (kereta) + Predicate (tidak terlambat) order. It’s idiomatic and polite.

Can I drop the subject Saya?

Not with berharap. Berharap kereta tidak terlambat sounds incomplete. If you want to omit the subject, switch to the particle of hope: Semoga kereta tidak terlambat.

What’s the difference between berharap, harap, and mengharapkan?

berharap = “to hope” (intransitive). Very common: Saya berharap…
harap = base verb. Saya harap… is shorter and slightly firmer. Bare Harap… often reads as an imperative “Please…”
mengharapkan = transitive “to hope for/expect,” usually takes a noun: Saya mengharapkan kedatangan kereta tepat waktu. Using it before a full clause is formal and can sound heavy in speech.

Why is it tidak and not bukan?

Use tidak to negate verbs and adjectives, and bukan to negate nouns/pronouns. Terlambat is an adjective, so tidak is correct. Example contrast: Ini bukan kereta saya.

What does the prefix ter- in terlambat do?

Here it forms a stative adjective meaning “late.” The prefix ter- has several roles (stative like terbuka, superlative like terbesar, accidental like terjatuh). In terlambat, it’s just the adjective “late.” Colloquial synonym: telat.

Is tidak terlambat the same as tepat waktu?

Close, but not identical. Tidak terlambat only says “not late.” Tepat waktu says “on time,” which is a positive target. You can say: Saya berharap kereta tepat waktu or … kereta tiba tepat waktu.

Are there other natural ways to express this hope?

Yes:

Semoga kereta tidak terlambat. (very common)
Mudah-mudahan kereta tidak terlambat. (also common, a bit more formal/old-fashioned in tone)

Do I need akan to express the future?

No. Indonesian doesn’t require tense marking. Saya berharap kereta tidak terlambat already implies a future situation by context. Adding akan (…tidak akan terlambat) just adds futurity/emphasis.

Should I add bahwa or agar after berharap?

Optional:

Saya berharap bahwa kereta… = “I hope that…” (bookish/formal).
Saya berharap agar kereta… = also formal, common in writing. In everyday speech, most people just say Saya berharap kereta… or use Semoga…

Is Saya berharap jangan terlambat acceptable?

Avoid it. Jangan is an imperative “don’t…,” not the right negator here. Use tidak: Saya berharap kereta tidak terlambat (or formal …agar kereta tidak terlambat).

Is kereta the same as kereta api?

In Indonesia, both mean “train.” Kereta is very common in speech; kereta api is fuller/more formal and frequent in writing or signage. Note: in Malaysian Malay, kereta means “car,” but in Indonesian “car” is mobil.

How would this sound casually vs. formally?

Casual: Aku harap kereta nggak telat. / Semoga kereta nggak telat.
Very casual (Jakarta slang): Gue harap kereta gak telat.
Formal: Saya berharap agar kereta tiba tepat waktu. / Kami mengharapkan kedatangan kereta tepat waktu.

How do I specify which train or make it plural?

Specific: kereta itu (that train), kereta ini (this train), or the service name: KA Argo Bromo.
Plural: use numbers/quantifiers: beberapa kereta, dua kereta. Reduplication (kereta-kereta) is possible but less common in everyday speech.

Can I say …terlambat tidak or move tidak to the end?

No. Tidak precedes the predicate it negates. Keep kereta tidak terlambat, not kereta terlambat tidak.

Is Kuharap kereta tidak terlambat okay?

Yes. Attaching ku- to the verb (kuharap) is correct but feels literary/poetic or casual in writing. In neutral speech, Saya harap/berharap… is more common.