The Government should encourage train operating companies to provide refunds for all pre-booked but unused tickets during the Covid-19 crisis, regardless of ticket category (eg the normally non-refundable Advance tickets).
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The advice to reduce unnecessary travel & cancellation of many events means that many people will not use their train tickets. Many categories of pre-booked tickets, eg "Advance" tickets are not normally refundable & only a partial refund is available if you are able to re-book for a later date.
Some people with such tickets might choose to travel just so as not to lose the money. In the interests of discouraging people from unnecessary train travel, full refunds should be provided.