There is nothing wrong with working as a train driver in a foreign country

There is nothing at all wrong with you working as a train
driver in a non-Muslim country, and we do not think that there are any
reservations about such work that would require you to refrain from doing
this job. Rather it is lawful provision, by Allah’s leave, for several
reasons:

Firstly:

The contract between you and the train company only has to do
with driving, and not with carrying or transporting prohibited items. What
matters with regard to contracts – from a shar‘i ruling perspective – is
what is mentioned in specific terms in the contract, which in this case is
entirely permissible work, namely driving the trains.

With regard to the curse on the one who carries alcohol, what
is meant is one who deliberately carries it and benefits from doing that, to
help the one who drinks it. All of these conditions are not applicable in
the case of driving trains or flying aeroplanes.

Secondly:

Seeing that the purpose of your work, according to the
clauses of your contract, is permissible, you are not responsible for asking
passengers about what they may be carrying, or to examine the goods that
they are carrying with them. You are not expected to ask them about that,
because what is being transported here is people; whatever they are
carrying, especially whatever is hidden or concealed of that, is a secondary
matter, that is not subject to any shar‘i ruling. Otherwise, people would go
through a great deal of hardship if every taxi driver – even in Muslim
countries – had to ask anyone who wants to ride with him about everything he
is carrying, for fear that he may have with him some cigarettes, for
example, or some prohibited material that is commonly found on smart phones
nowadays, and other similar scenarios where it could be very difficult if
sharee‘ah were to require that. Rather, if we were to require this
burdensome duty of train drivers, pilots and taxi drivers, then we would
also have to require them to ask everyone about the destination to which he
is headed and the purpose for that journey, so that if it was for unlawful
purposes he would refuse to drive or fly the plane in that case. We do not
believe that anyone would hesitate to regard such requirements as
inappropriate, which indicates that this requirement itself is also invalid
and there is no need for all that. This confirms that sharee‘ah could not
have enjoined any such hardship so long as the nature of the job in question
– namely driving – is permissible in principle, praise be to Allah.

Thirdly:

You mentioned in your question that you do not know what the
train passengers are carrying, the reason being that the driver is usually
separated from the passengers in a compartment in the front carriage, which
makes it difficult for the driver to know what passengers are carrying,
especially when it is in closed bags. Therefore there is no requirement or
obligation concerning this issue in particular.

Fourthly:

If we reflect upon the fiqhi principle on which there is
consensus in general, that “a secondary matter may be overlooked where a
primary matter could not be overlooked,” this reinforces what we said about
the ruling which says that it is permissible. That is because the issue of
prohibited items that some people may carry is a secondary matter and is
not  intended as part of the job description (of a train driver), and a
secondary matter may be overlooked if the primary matter is permissible. The
primary matter in this case is the transportation of people and what they
may be carrying of personal items.

The fuqaha’ expressed this principle in many ways, some of
which are more directly connected to the question in this case, such as the
view of Imam as-Sarkhasi (may Allah have mercy on him): It may be
permissible to include in a contract that which may not be permissible if it
were the main focus of the contract.

End quote from al-Mabsoot (11/179).

And al-Qadoori said: Something may be included as a secondary
matter pertaining to a contract that cannot be made the main focus of a
contract.

End quote from at-Tajreed (8/3792).

Imam as-Suyooti (may Allah have mercy on him) said: Anything
may be overlooked if it is something implicit and secondary that may not be
overlooked if it is the main focus of a contract.

End quote from al-Ashbaah wa’n-Nazaa’ir (p. 120). See
also: Haashiyat al-‘Attaar ‘ala Sharh al-Jalaal al-Muhalli ‘ala Jam‘ al-Jawaami‘
(2/160); Mu‘allimah Zaayid li’l-Qawaa‘id al-Fiqhiyyah wa’l-Usooliyyah
(11/531).

The example mentioned by the fuqaha’ is the permissibility of
renting out something to someone who will use it for a permissible purpose,
as in the case of a Christian who rents a house from a Muslim to live in it,
but he may commit some unlawful deeds in his house as a secondary matter,
not as the purpose for renting the house. This may be overlooked, and there
is no sin on the one who rents out the house; rather the sin is only on the
renter, and the one who rents it to him has the right to stop him doing that
if he comes to know of it, as was stated by the Hanbalis.

Imam Muhammad ibn al-Hasan ash-Shaybaani (may Allah have
mercy on him) said:

If a dhimmi (non-Muslim living under Muslim rule) rents a
house to live in, there is nothing wrong with that. But then if he drinks
alcohol in that house, or he worships the cross, or raises pigs there, there
is no sin on the Muslim for that, because he did not rent it out for that
purpose.

End quote from al-Asl (4/17).

None of the scholars – as far as we know – ever said that it
is prohibited to rent out permissible things for permissible purposes, as
stated in the contract, for fear that the renter might use the rented thing
– occasionally and as a secondary activity, and not as the primary purpose
of the rental – for some unlawful purpose. If anyone said that, it would
cause a great deal of hardship to people.

What is mentioned in the question comes under this exact
category. There is no blame on the driver of any means of transportation;
rather the sin is on the one who carries or transports the unlawful item, or
commits the unlawful deed.

Fifthly:

The scholars are unanimously agreed that whatever could be an
indirect means to something unlawful, we are not required to block those
means, so that this will not cause hardship in other aspects of life. For
example, they said that it is not permissible to say that it is prohibited
to cultivate grapes in general, even though cultivating grapes will
undoubtedly lead to the making of wine, but that is an indirect outcome, and
grapes are usually grown for permissible purposes, namely to supply them to
people who want to eat grapes in a permissible way. In such cases the rule
of blocking means that may lead to evil is not applicable.

Imam al-Qarraafi (may Allah have mercy on him) said:

The ummah is unanimously agreed that there are cases where
the means are not to be blocked if they may indirectly lead to something
evil, and he gave examples of that: the cultivation of grapes is a means
that leads to producing wine, but no one says that it should be disallowed
for fear of producing wine.

End quote from al-Furooq (2/42).

Similarly, in the case under discussion here, forbidding
people to drive public transportation for fear of what “some” passengers
“may” be carrying of unlawful items – whether that is alcohol and the like –
would cause people a great deal of hardship in their daily lives, and would
deprive them of permissible means of earning a living, with no clear reason.

Conclusion:

There is nothing wrong with working as a train driver in a
non-Muslim country, unless there is specific mention in the contract of
carrying some unlawful things, in which case it is forbidden to be party to
a contract that specifically mentions certain unlawful items to be
transported.

And Allah knows best.