Absolutely ask, albeit politely.
You may have some clues already about whether or not such a request is likely to be granted, based on whether or not the position you are interviewing for pays for relocation expenses or not.
But if there is any doubt or uncertainty, it does not hurt to ask whether or not they would be able to provide any reimbursement for travel expenses since the interview is quite a trek from where you live. If reimbursement is available, you may want to follow up with clarifying questions about whether the reimbursement only includes the bus tickets or whether it includes other expenses such as meals and hotel costs.
Point to keep in mind:
Do keep in mind that if the company isn't paying your travel expenses to get to the interview it probably won't pay to relocate you either, so you may opt to decline the interview if you can't afford to get there or to move.
Of course, even if the employer will not reimburse you, the job search expenses might be tax deductible, even if you do not itemize your taxes.
In any case, I would ask.