How can I import a large list of tasks (.csv or .txt) into Trello and convert into cards that are part of a list
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.
There is now a free power-up which lets you import multiple cards from Excel/CSV files. This includes the ability to add all the standard Card fields and any Custom fields you have set up. You can try it out here Trello Import by Blue Cat.
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)