… originally, duties of the common carriers were simply those of bailees in general, coupled with the liabilities attached to the exercise of a public calling and were not peculiar to common carriers only but were regarded as the custom of law of common hoymen, or lightermen, etc., according to the business of the party concerned.
J Spurling Ltd v Bradshaw  2 All ER 121 at p.125 Denning LJ said:
A bailor, by pleading and presenting his case properly, can always put the burden of proof on the bailee. In the case of non-delivery, for instance, all he need plead is the contract and a failure to deliver on demand. That puts on the bailee the burden of proving either loss without his fault (which would be a complete answer at common law) or, if the loss was due to his fault, that it was a fault from which he is excluded by the exempting clause.
Levison and another v Patent Steam Carpet Cleaning Co Ltd  3 All ER 498, per Lord Denning M.R. at p.505:
…in a contract of bailment, when a bailee seeks to escape liability on the ground that he was not negligent or that he was excused by an exception or limitation clause, then he must show what happened to the goods. He must prove all the circumstances known to him in which the loss or damage occurred. If it appears that the goods were lost or damaged without any negligence on his part, then, of course, he is not liable. If it appears that they were lost or damaged by a slight breach, not going to the root of the contract, he may be protected by the exemption or limitation clause. But, if he leaves the cause of loss or damage undiscovered and unexplained, then I think he is liable: because it is then quite likely that the goods were stolen by one of his servants; or delivered by a servant to the wrong address; or damaged by reckless or wilful misconduct; all of which the offending servant will conceal and not make known to his employer. Such conduct would be a fundamental breach against which the exemption or limitation clause will not protect him.