They are railway station announcements in English. The time always refers to the departure of the train.
They are neither dialect or slang. To be employed as an announcer it would be necessary for the person to speak clear English. Dialect is a regional or localised way of speaking a language... Cockney is a dialect of London, but of course the announcer may be from another part of the UK or indeed another country. Slang is never used in formal situations such as this. It is a method of communicating between people who understand the usage and can make sense only to people who are familiar with the expressions. A Cockney accent uses common english but the accent makes it sometimes difficult for others to understand. Cockney rhyming slang on the other hand uses the same accent but possibly incomprehensible use of figures of speech, which are perfectly clear to those who know the slang. It is common in different areas of work or the army for example. Rhyming slang is particularly colourful. I suggest you try searching for 'slang' and 'accent'.
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