URGENT: What's the difference between London Overground and London Underground?

Yes, I'm sure it's that one provider is underneath the roads and the other isn't. But what about South Ealing Underground Station is which is "an Underground station" YET its platform is outside as if it were a Southeastern railway station. Why is this? So, in short, can someone please kindly... show more Yes, I'm sure it's that one provider is underneath the roads and the other isn't. But what about South Ealing Underground Station is which is "an Underground station" YET its platform is outside as if it were a Southeastern railway station. Why is this?

So, in short, can someone please kindly explain the principles of the following:

- Southeastern
- London Overground
- London Underground

Thanks
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