How can I import a large list of tasks (.csv or .txt) into Trello and convert into cards that are part of a list
Most of the solutions I saw here were pretty technical. Here is one that is really easy to do. You could use this Zap to automatically create Trello Cards from Google Docs spreadsheet rows.
The idea is that once you make this Zap, you can paste your CSV into the a Google Doc and run it through into Trello.
I think there is a limit of about 200 items per 10 minutes when importing stuff across, but normally you won't hit that.
I've created and shared a Google Spreadsheet which you can use to import multiple entries. It's geared towards a Scrum product backlog, but you could customise it if you want. Details here: http://www.littlebluemonkey.com/blog/online-scrum-tools-part-3-upload-existing-product-backlog-into-trello/
Another option is to use the Import Wizard, by Import2 (disclaimer, I work for Import2). This will import your data directly from CSV into a list in Trello.
I assume you got this TXT or CSV file by exporting data from some other system. in some cases you can skip this intermediate step and transfer tasks between systems directly. one of the products that can do that is TaskAdapter: http://www.taskadapter.com.
e.g. here are steps for Trello and Atlassian JIRA integration: http://www.taskadapter.com/user-guide/trello/
or for JIRA and Microsoft Project: http://www.taskadapter.com/user-guide/atlassian-jira/atlassian-jira-and-microsoft-project-integration-step-by-step-guide/
(disclaimer: I am owner of TaskAdapter product)