Can 'obsolete' and 'defunct' be used interchangeably?

'Obsolete' is defined as

No longer produced or used; out of date.

and 'defunct' is defined as

No longer existing or functioning.

The meanings sound close to each other though the latter should be more formal. Are there situations in which native English speakers use them interchangeably?

An example from OALD:

The LP was made defunct by the arrival of the CD.

Solution 1:

Defunct refers to the company making a product and obsolete refers to the product itself.

For example, consider Netscape Navigator:

Netscape Navigator was a proprietary web browser, and the original browser of the Netscape line, from versions 1 to 4.08, and 9.x. It was the flagship product of the Netscape Communications Corp and was the dominant web browser in terms of usage share in the 1990s, but by 2002 its use had almost disappeared. source

Netscape Communications Corp is defunct as the company is no longer in business. It was bought by AOL. In contrast, its Netscape Navigator is obsolete. This browser cannot handle many modern HTML conventions.

The OP cited an example of a defunct rail line. Lexico.com

the now defunct Somerset & Dorset railway line

This means that the line is no longer being used (and perhaps stopped carrying passengers only a week ago). It is not functioning at the moment, but could be in the future. That defunct railway line could be dusted off and recommissioned next week. While not broken, it is not being used in its original capacity.

If the manufacturer of the train were out of business, then it would be defunct, as well.

The engines on that rail line could be obsolete steam locomotives. But you would not call the engines defunct.

Solution 2:

The answer is in the definitions you have given. 'Obsolete' is No longer produced or used; out of date. - this does not mean it is non-functioning. 'Defunct' means it is broken - No longer existing or functioning.

Here's an example:

• A perfectly working iPhone 5 could be called obsolete - it is out of date, since newer models exist.
• An iPhone XR, the latest at time of writing (hah), could smash as soon as you open the box. It is now defunct, but not obsolete. It would become both defunct and obsolete when you replace it.