That would depend on the company's HR/travel policy.
Agreed.
Exactly, when an employer/contractor/benefactor pays for your ticket (either up-front or by claiming back expenses). The particular travel I'm thinking of was on behalf of a charity so I'm thinking both morally and legally, whether they belong to the passenger or to the charity who ultimately paid for the travel.
I would expect different treatment depending on whether the person suffering the delay was a volunteer or a paid worker.
Volunteers give their time freely to the charity, often in return for repayment of travel expenses. I would not expect a charity to seek to recover the value of rail travel vouchers from a volunteer who has been forced to give more time than they planned to the charity as a result of train delays.
For paid staff of a charity the position is more complex. I think the principle should be that treatment is consistent and, if at all possible, has been agreed by the representatives of the charity's staff. The larger charities tend to mirror the conditions for public servants.
One government agency I'm familiar with has a different approach depending on whether the worker is delayed getting home. If not, for example delays on the outward leg of a day return, the value of the voucher is deemed to belong to the agency. If there is a delay getting home, the worker may choose between claiming a time credit on their flexible working hours or retaining the voucher.