Если не будет билетов на поезд, у нас есть запасной вариант: поехать на автобусе.

Because the sentence is talking about a possible future situation.

• нет билетов = there are no tickets now
• не будет билетов = there will be no tickets

In Russian, an если clause can use the future tense normally when the condition is about the future:

• Если будет время... = If there is time...
• Если не будет билетов... = If there are no tickets / If there won’t be any tickets...

This is different from English, where we usually say If there are no tickets, not If there will be no tickets.

After existential negation such as нет, не было, and не будет, Russian usually puts the noun in the genitive.

So:

• Есть билеты = There are tickets
• Нет билетов = There are no tickets
• Не будет билетов = There won’t be any tickets

That is why you get билетов, not билеты.

Here на поезд is part of the fixed pattern билет на + accusative, meaning a ticket for something.

So:

• билет на поезд = a train ticket
• билет на автобус = a bus ticket
• билет на концерт = a ticket for a concert

It does not mean physical movement onto the train here. It just means the ticket is for that train journey.

Because у нас есть is the normal, everyday way to say we have in Russian.

Literally, у нас есть means something like at us there is, but idiomatically it simply means we have.

• У нас есть запасной вариант = We have a backup option

The verb иметь does exist, but it is often more formal, bookish, or used in special contexts. In ordinary speech, Russians usually prefer у кого-то есть.

Because the sentence means that the backup option already exists now.

The idea is:

• If there are no train tickets, we already have a Plan B.

So у нас есть запасной вариант is a present fact. Russian often does this when the option, plan, or possibility exists already.

If you changed the wording to a future form, the meaning would shift slightly toward we will have a backup option at that time.

It means backup option, fallback, or Plan B.

• запасной = spare, reserve, backup
• вариант = option, variant, possibility

So запасной вариант is a very natural way to say that there is another plan available if the first one does not work.

The colon introduces an explanation of what the backup option is.

So the structure is:

• у нас есть запасной вариант:
  поехать на автобусе

In English, this works similarly to a colon in We have a backup option: take the bus.

Russian uses the colon here to mean namely or that is.

Because the infinitive is being used to name the action that serves as the backup option.

Russian often does this after nouns like:

• план = plan
• идея = idea
• возможность = possibility
• вариант = option

So:

• вариант: поехать на автобусе = the option of going by bus

English can do something similar:

• Our option is to go by bus

The subject is not repeated because it is understood from context: we are the ones who would go.

This is about aspect.

• ехать is imperfective
• поехать is perfective

Here поехать is natural because the sentence is talking about one concrete trip as a complete action: to go / to set off by bus as a specific solution.

Very roughly:

• ехать focuses more on the process of traveling
• поехать focuses more on starting or carrying out the trip as a whole

In a sentence about a backup plan for one journey, поехать sounds more natural.

Yes, автобусом is also possible, but на автобусе is very common and natural.

Russian often uses на + prepositional with public transport:

• на автобусе = by bus
• на поезде = by train
• на трамвае = by tram

You can also sometimes use the instrumental without a preposition:

• автобусом
• поездом

Both can mean by bus / by train, though на автобусе is especially common in everyday speech.

Yes. Russian word order is flexible, and that version is also natural:

• Если не будет билетов на поезд...
• Если билетов на поезд не будет...

Both mean the same thing. The difference is mainly emphasis and flow.

• Если не будет билетов... puts the negated existence first
• Если билетов на поезд не будет... highlights tickets for the train

Because Russian uses case endings, word order often changes more freely than in English.

More or less, yes — that is close to the literal structure.

A word-for-word breakdown would be:

• Если = if
• не будет = there will not be
• билетов = of tickets
• на поезд = for the train
• у нас есть = we have
• запасной вариант = a backup option
• поехать на автобусе = to go by bus

But in natural English, you would not translate it so literally. You would say something like:

• If there are no train tickets, we have a backup option: go by bus.
• If there aren’t any train tickets, we can take the bus instead.

So the Russian grammar is different from English, even though the overall meaning is straightforward.